I finally beat Impossible on my second playthrough. It also unlocks
I saved every turn. It was so hard that I personally think that beating Impossible Ironman is undo-able without abusing glitches. And even then, you'll have to dashboard every turn.
I do not recommend starting on this difficulty if this is your first time playing.
Besides getting 1 shotted every turn, you have a huge money and panic problem.
Here's what to look forward to at Impossible difficulty
1) Most aliens have more health than normal difficulty.
Eg; Thinman's hp. 3 for normal/easy. 4 for Classic. 6 for Impossible!!! Even a jump from 3 to 4 is huge, as this means you can't kill them with 1 grenade.
2) Aliens also have ridiculous accuracy and crit a lot (2 shots and you will die, even with the best gear. I get hit for 15-17 dmg regularly near the end. Tip: Reload if you get shot).
3) Your satellites only get half the income.
4) You are not gifted with an officer training school at the start; this means you're stuck with only a 4 man vanilla squad for a LONG time.
5) Panic increases from failed missions are doubled in Impossible.
6) other little details like longer lasting poision, your archangel armor lasts half as long in flight, etc etc.
Luckily, research time is the same (I never built a lab and I got along fine).
For me, the difficulty was VERY hard for the first month. Second month was easier as my guys were leveled up. By the 4th month came I had every gear upgraded to the max and it was a cakewalk (relatively). I only lost 3 countries in the first month; South America + India. And never lost any since. Aim to get North America next (for the bonus) if possible as you will need planes to defend your satellites.
I started in Africa for the +30% funding bonus. I never built a lab the entire game. And I used all my money ONLY for satellites and uplinks at first; however, I built the Alien Containment and arc thrower near the 2nd month to speed up my research and to sell the weapons the stunned aliens will drop. Additionally, I never built a foundry/officer-school/plane/etc until I maxed satellites.
Skip the SKIV and the laser lines. You have no money to build lasers and SKIVs even if you research it anyways. Research armor as you start (but don't buy any), and capture a Muton as soon as you can (!!!), interrogate him, and then get every plasma gear ASAP so you don't get destroyed. Interrogating a Muton gives you the plasma research bonus so don't start the plasma research too early. FYI, a plasma pistol does more damage than all your starter gear.
Here's a list of the research topics so you can quickly get to plasma.http://www.ufopaedia.org/index.php?title=Research_(EU2012
My squad was running around unarmored and with their starter gear until the third month. And I survived by absolutely abusing the shit out of the save/load system.
My 4 man squad survived for 3 months on starter gear consisted of 2 assaults, 1 heavy (rocket+shredder), 1 sniper (squadsight+doubletap). Support is freaking useless. Note: Your starter squad has only 3 health. BUT if you hire new guys, they start with 6 health (pre-patch). To abuse this, I hired 5 guys (with 6 hp) immediately after the first mission and fired the old guys (with 3 hp). The extra hp makes a big difference, it's tempting to play without the patch.
I won't go over the basics to winning a mission. There's just too much, but you can check here. http://www.gamefaqs.com/boards/655783-xcom-enemy-unknown/642...
There's lots of little tricks to win a mission, if you've played the original X-Com games, you'll know most of them already. But here's a special trick to this game that will help veterans of the X-Com series already familiar with the basic tactics.
The KEY to surviving everything without plasma is the 2 assault guys. They are the only unit that can kill EVERYTHING in the game in 1 turn. NO other unit can do this!!!
Usually mobs spawn in groups of 2-3 aliens (unless you spawned more than 1 group which usually means you're screwed). Here's how to take care of a group of 2-3 aliens. Your assault guys can each solo 1 alien, and the last alien can hopefully be killed by your sniper and heavy. You can kill an alien easily by using this tactic. This is my fail-proof tactic, so listen up.... your assault hero is to charge in (lightning reflex),sprint to the enemy (run and gun), fire 2 shotgun shots point blank (rapid fire + killer instinct). No alien in the game can survive this tactic, even on Impossible difficulty!!! After you've killed the alien, pray that you survive till your next turn starts (pick close combat specialist skill), but most of the time you've killed everything around you. Later on in the game you can use ghost armor to abuse this tactic even more.
In the beginning, most missions will only have 8-12 aliens. You heavy weapon guys should have 2 rockets (with shredder upgrade), which should kill 2-3 aliens with each rocket. The rest of the baddies you will need to kill with grenades and your skills.