++Introduction++
Obviously if you're looking for Impossible Mode tips, you've played Dead Space before. But I would recommend playing Impossible Mode right after you play through a lesser mode of Dead Space. There's nothing wrong with playing on a lesser mode and then waiting a few months and then come back to play Impossible Mode, but playing through the game will familiarize yourself with where items are and knowing when to expect certain events such as getting gangbanged by a bunch of necros. In other words, being caught by surprise will be kept to a minimum.
There are a couple of ways to play through any mode of Dead Space. I've heard people saying stasis upgrades is the way to go with Impossible Mode and this does work well, but I prefer a more aggressive approach. There's something extremely satisfying about Force Gun blasting groups of necros and then picking them apart with the Line Gun and Plasma Cutter. Stasis is really only useful at select times and through the first couple of chapters. Your default stasis is more than enough to get you through Impossible Mode. There are tons of Stasis Packs throughout the game, but these are a very good source of credits and allow you to buy more nodes for upgrades.
Remember that upgrading the Capacity of weapons will refill your ammo. Same goes for your Rig: upgrading HP and Stasis will refill those attributes to max as well.
++Weapons++
1. Plasma Cutter: default weapon
2. Line Gun: you can buy this the first time you visit the store - get it at least by chapter 2 because that's where the Line Gun ammo starts dropping.
3. Force Gun: The schematic is in chapter 4.
These three weapons cover everything you need: Pinpoint accuracy for dismembering (Plasma Cutter), bosses (Plasma Cutter), significant damage against multiple limbs (Line Gun) and, most importantly, knock back power when you're in close proximity of multiple necros (Force Gun). By concentrating on upgrades for these three weapons, you have three powerful weapons that fit any situation and not many weapons that excel at nothing.
1. Plasma Cutter: This weapon is good for everything but excels at nothing. I often switched to the Plasma Cutter after I blew the legs off a necro with the Line Gun to finish the necro off.
-Key Plasma Cutter Upgrades: Speed, Damage and Reload. Capacity should only be upgraded when you're out of ammo and on the last boss (at which time you should upgrade and as close to max Capacity as you can). Work the node circuits for Speed first and the Damage will come naturally. Quit upgrading it once you max your damage until you need to upgrade Capacity to get some free ammo.
-Effectiveness: Great against ALL necros and bosses.
2. Line Gun: This weapon is good from the start and is great without upgrades until chapter 6 or so. After Chapter 6, it's still good, it just requires more ammo. It blows multiple limbs off necros without having to adjust your aim and this gives you a major, major advantage over necros like Black Slashers and Regenerators.
-Key Line Gun Upgrades: Damage and Reload; Capacity will come while upgrading Damage and Reload - Mine and Width are pointless. Too much Width actually makes this weapon more difficult to use because you cannot see your complete target area at close range and will often cause the waste of ammo. This weapon does NOT have to have fully upgraded Damage to be a very good weapon. In fact, only 6 nodes on this entire weapon are actually needed. The two immediate Capacity node slots are nice if you're out of ammo before you get the Force Gun.
-Effectiveness: Great against anything with easy access to limbs (legs preferably), or the fat necros that spawn babies from a distance. This weapon is especially effective against Slashers, Lurkers and Regenerators.
3. Force Gun: Awesome push back power. This weapon also has a decent alternate firing mode - it fires a mine that bounces around on the floor and it does great damage to large groups of necros.
-Key Force Gun Upgrades: Speed, Reload and Damage. You really don't have to upgrade this weapon at all, but you will definately see the potential of this weapon if you do upgrade it. Get your Rig maxed out on HP and Plasma Cutter maxed out on damage before upgrading anything other than the first Speed and Damage slots on the circuit board of this weapon.
-Effectiveness: Great against every necro in the game (though avoid shooting the fat necro that spawn spiders if you can) and especially good at cleaning up spiders and those Crawlers that come in packs (looks like a skull with tentacles) if you don't use precise shots against the Divider (tall necros whose moan is very disturbing). As mentioned previously, it has great knockback power. When this weapon knocks a necro back, you have plenty of time to draw the Plasma Cutter and dismember limbs if you have to. It one-shots groups of spiders that spawn from the fat necro (sorry, I don't know the name for this necro) if you make a bad shot. Use it enough and it kill the necro.
The Line Gun and Force Gun will be your main weapons throughout the game. They take little skill to use which makes Impossible Mode easy for even the most casual of gamers. Use one until you get low on ammo then switch to the other. By the time you switch back again, you should have a healthy surplus of ammo for the other weapon. I didn't have to buy any ammo for either of these weapons until chapter 12 when I ran out of both and then I only bought Force Energy because it's cheap and is really a better weapon due to it's knock back power.
++Rig++
Level 1 - default Rig
Level 2 - bought at the first store - 10k credits
Level 3 - schematic found in Chapter 4 - 20k credits
Level 4 - schematic found in Chapter 7 - 35k credits
Level 5 - schematic found in Chapter 10 - 60k credits
Total Cost of Rigs: 125k
Upgrade HP first and as you need HP (honestly, there are enough medipacks in the game to tide you over, but the first chapter or two may prove difficult). Never upgrade Air. Stasis upgrades are not needed, but it's up to you. Be prepared to use 3 nodes before getting to upgrade anything for Stasis though. IMO, these 3 nodes are far more valuable elsewhere.
When running across the topside fo the ship with asteroids pelting you, take a Medium Air Can with you just in case. There are plenty of O2 stations here, so you'll likely not even need it. To make it even easier, there are Save Stations in most spots before you even go into the vacuum of space.
It takes 12 Power Nodes to fully upgrade the HP attribute of the Rig. These HP upgrades are more for basic content of the game as the bosses are very easy. As cool as the bosses in Dead Space are, I was more than slightly disappointed about their difficulty level. I was looking forward to some epic boss battles...
Stasis upgrades are not needed. The only place I would suggest even thinking about a Stasis upgrade is when the Regenerator attacks in the chapter where you have to push the beds out of the way to get to a key item. Even then it is not remotely needed. However, if you feel you must upgrade your Stasis Module, get just one Duration. Energy is pointless
++Power Node Usage++
If my count is accurate, there are 34 Power Nodes scattered throughout the 12 chapters of Dead Space.
-The Rig will require at least 12 Power Nodes for HP upgrades.
-The Plasma Cutter will require 18 to fully upgrade it.
-There are 4 Power Node Doors you will benefit by opening.
That leaves a grand total of 0 Power Nodes, so buying these nodes will go a long way in helping you on your quest. Buy them early and jack up the damage of your Plasma Cutter and/or HP on your Rig. Just remember to look at the Rig costs above before spending every last credit on Power Nodes.
++Power Node Doors++
The only Power Nodes you should be using on doors are in Chapters 4, 7, 10 and 12. 10 and 12 are your call depending on your ammo and health situation. Chapter 10 and 12 node doors won't get you any credits, but there are a nice supply of ammo in those doors that outweigh the cost of a Power Node (if bought). The other rooms I listed have items you can sell to garner more than the 10k credits it cost to buy a Power Node from the store.
TIP: The Power Node door in Chapter 12 can ONLY be opened AFTER the lockdown is lifted! This is very important otherwise you'll waste a node!
++Final Boss++
Take a ton of Plasma Cutter ammo with you and 1 or 2 Force Gun ammo with you. You don't need anything in your inventory but ammo. You don't even need medipacks. Use the Plasma Cutter in horizontal firing mode.
-STAGE ONE: Targets are the 5 yellow bulbs that look like eyes. It takes approximately 10 shots from a fully upgraded Plasma Cutter to destroy a bulb. The boss will slam it's tentacles on the ground at you. These are easily avoidable. Just strafe left/right while keeping the camera focused on the boss everytime it swings a tentacle. Once you destroy 3 bulbs, the boss will pick you up with a tentacle and move you towards his mouth.
However, don't just shoot randomly. Shoot the two bulbs *ON THE BOSS RIGHT SIDE* 7-8 times each to weaken them and then destroy the 3 bulbs on the left side of the boss. The reason for this is that your aim is inverted (swap your targeting axis around from the /pause menu if you're having trouble aiming here) and it's a tad more difficult to aim. but, since we shot the two right bulbs 7-8 times each, it will only take 2-4 shots to destroy those two bulbs. It's easier to target the bulbs further away from you as you get closer to the boss' mouth and that is why we chose to leave the two right side bulbs to destroy last.
-STAGE TWO: The boss will throw you back to the ground (equip your Force Gun at this point). It will also start spitting cocoons at you. These cocoons will hatch the fat necros that spawn spiders. Fire at these necros if you like, but the boss will kill them when it slams it's tentacles at you. Use the Force Gun to destroy the spiders that spawn after the fat necros are destroyed.
Reequip the Plasma Cutter.
The boss will use 4-5 tentacle slams in a row (it does this even while the necros are hatched from cocoons). Again, easily avoidable if you strafe left/right (use LB to run while you strafe if need be). Silly boss, why telegraph every attack you have? After these tentacle slams, the boss will open up it's rib cage exposing more yellow bulbs to destroy. If you destroy one, it will close it's rib cage and use the 4-5 tentacle slam attack again, Just repeat stage two tactics for the win. Stay away from the boss as it falls as you do receive damage if it falls on you before falling back into the crater.
++General Information and Tips++
1. Clear the room of all possible necros before picking up item.
2. Stomp all dead bodies in a room where you know there will be an Infector (flying carpet-looking necro). These necros create Stalkers buy turning the dead crew of the Ishimura. If the dead crew have no limbs, they can't be turned leaving on the Infector (and any other default necros in the room) to deal with.
3. Stomp boxes, don't shoot them.
4. Melee switches and fuse boxes, don't shoot them.
5. Use Telekinesis to nab all the items/boxes within reach from a room before entering it.
6. Keep one Stasis Pack in your inventory at all times. Keep one in the bank in case you have to use that Stasis Pack and sell the rest.
7. Sell all Small Medipacks unless you have to top off your health.
8. Sell all ammo from all weapons other than the 3 listed above.
9. Buy Power Nodes. You should be able to buy quite a few nodes since you're selling a a lot of your loot.
10. Using Telekinesis to shoot explosive canisters to kill necros won't get you any loot. So only do this when you're low on ammo or in large groups of necros. That said, always carry one with you via Telekinesis when you see one.
11. ALWAYS pick up items in a room before grabbing a key item. Chance are that once you grab that key item, you're about to get bumrushed (yes, I did go there... I used that late-80's term that wasn't even funny in the late-80's).
11a. Hole-up (when possible) in rooms where you have to grab a key item. There aren't many places to hole-up in Dead Space due to necros coming from everywhere, but if you can find a corner to put your back into, and you have a decent field of vision, you're survivability just increased 10 fold.
12. Don't get discouraged. Impossible Mode gets easier as the game progresses as you upgrade your gear.
13. Use Stasis to get out of tight spots. If you're being mauled, by all means, slow them down. However, the Force Gun should be more than adequate for this situation.
14. Make sure you have enough credits to buy the next level of Rig after you find the schematic.
15. Keep your weapons fully loaded every time you get the chance to reload them UNLESS you plan on using a node to upgrade Capacity at a work bench.
16. Do not upgrade Kinesis at all.
17. In the area where the Regenerator drops from the ceiling after moving all those beds and blocking yourself in, pick up a lantern with Telekinesis and set it down to mark your path of escape back through the beds.
That's about it. Hope this helps at least one person out and is not too long for people to not read. I know it's long, but please remember the achievement is for a mode of playing through the entire game. So, naturally, it's going to be long. Good luck!
EDIT 03/18/11
I just want to add something real quick (okay, so it's not "real quick" hehe):
While I absolutely appreciate the thumbs up from my fellow gamers that enjoyed my solution, please don't think that you have to thumb down the other solutions if you happen to think mine is better. If you think a solution is bad, then give it a thumb down. Or, better yet, contact the author and let he/she know that they should improve their solution and give them time to improve it before thumbing them down. I think you'll find that many solution authors are more than willing to improve their solutions because we write them to help out other gamers. There are other really good solutions for this achievement, they just offer alternative ways of getting the achievement. When we have more methods we have to achieve something we appeal to broader the audience of gamers and that is a very good thing: If one solution doesn't work, you have others to choose from!
I personally do not look at solutions-writing as a competition. I look at it as a community of gamers helping other gamers out. So, the more solutions we have that give different ways to get an achievement or that refine and tweak other methods already given, the better off we all are.
Anyway, I apologize for adding to the already lengthy solution (those that know me know I talk a lot hehe), but it seems that some of the other solutions for this achievement are garnering a lot of thumbs down when they don't deserve them. Thanks for reading!
EDIT #1:
Bardan Jusik asks if going for both One Gun and Epic Tier 3 Engineer achievements at the same time is possible.
My answer and opinion: Yes! Very possible and very easy for some people!
However, if using my solution AND going for the One Gun achievement at the same time requires tweaking my solution so that it covers all parameters of BOTH achievements. Some tweaking is common sense such as only using Power Nodes on the Plasma Cutter and the Rig. This fact alone will make the first few chapters of Dead Space easier AND you'll have plenty of credits to upgrade your gear sooner due to selling so much loot.
Some require a bit more thought, but they make sense when you actually sit down and think about the differences. In my personal opinion, if going for both these achievements at the same time is the ONLY time I'd upgrade Stasis. Slowing down Necros makes using the Plasma Cutter child's play.
Again, these suggestions are only if you're using my solution and going for One Gun. There are many ways to go about it, but these would be the simplest for me personally.
Good luck!