Okay, so first off;
You need to eat (meat or carbs) drink (water, soda or caffeine) and sleep regularly. You gain skill and stat debuffs that get worse until you die.
Companions dont fall in combat. They die.
You can only save on your ship. Autosaves happen outside Transition locations (entering/exiting Edgewater, for instance)
Certain perks end up being deadly.
Fast travel only works to your ship.
Seems scary, but its doable.
(AUTHORS NOTE - updated. I Helped Phin, the board, and finished the game at lvl 26 to see the min combat requirements.)
Rough enemy levels
Edgewater - lvl 1-7
Roseway - 10-12ish
Monarch - 14-18
Byzantium - 16-19
Hope - 20-21
Tartarus 25-26
My character during creation had temperment + 3 (7.5hp regen a sec)
-1 charm (didnt use companions, - to rep isnt noticable)
35% headshot dam with perception (dex for reload is a good choice too)
First off STEAL EVERYTHING YOU CAN. You will get ammo, guns, armor, food. Duh right? Well you can sell items, or more importantly break down cheap gear, as trust me you will be repairing a LOT. I left Edgewater with 12,454 bits. You read that right. Klepto to the top.
Break down most armor and any weapon under 70 bits, as you get more parts than the bits buy.
I used rifles and machine guns. I used Phins Phorce on Tartarus, otherwise I just used normal guns outside of mechs. When applying mods, the triggers switch what mods you apply where, so my electric gun was an assault rifle, the range of a shock cannon will kill you.
I went heavy armor. The stat boosts of light and medium dont help with damage, just take it off real quick if you need your 5-10 hack/lockpick skills back. Trust me you get plenty of skill points to play how you want. Respec on your ship if you need to (left most ladder as you enter your ship and take a left, near the workbench)
*Even with (endgame) 77 + 5 with mod body, 79 hat, + 5 perk, + 10% perk, I could only take a few shots before I had to hide/run. The idea of LESS armor on Tartarus scares me.
You want 20 sneak for gun crits, enough engineering to repair without a bench, enough science to tinker, and decent speech skills. Melee is useless, I ran solo so never had companion skills, and I dumped into medicine for heals. Add to science later level as the endgame boss is a mech who swarms drones, science = more electrical damage.
I had, for perks, 50% health, all the headshot damage I could get, more and faster recharging TTD, armor plus the perk 3 armor, 25% solo damage, 15% health for kills, more backpack. Really the rest werent as necessary, fillers like half consumable weight and faster run.
*+10 dialogue skills did help at times, consider it 3 lvl worth of skills.
*-25 durability loss is great early on, I ended the game with 847 wep and 589 armor parts though, take that how you will.
TINKER ULTRA WEAPONS AND ANY SCIENCE YOU WANT.
Ultra is the last stage of weapons and through testing, a mark 2 WONT hit the damage of an ultra, plus, tinkering costs x2 with each stage, so, leveling an ultra hunting rifle (lvl 17) to be usable on Tartarus would have cost me over 30k bits. YEAH.
HIT AND RUN. Enemies take three days to respawn, so take your time if you have to. You *cannot* get swarmed. RUN. Exit a building, retake an elevator, anything. Spray n' pray, I ended the game with 3k of every ammo. TRUST ME, you get PLENTY.
*Revisiting enemy spawns, in theory, would let you farm for parts, ammo, bits and food. Worth noting if you feel like you need a boost, enemies DONT level with you.
Perks For Flaws Are Suicide. Extra Damage Isnt Worth It. The ONLY flaws I took were phobias to raptorbastards and mandtitskum. The raptorwahoozits were relatively easy to kill and I never fought mandyqueens. I ran. I got out of their agro. Even to the comm tower on monarch I just RAN.
Storywise, I helped Phin, keeping board kills to a minimum. When Sophia offered to let me swap sides (after stealing the chems), I planted her signal. This let me wipe board notoriety, so even when jumping the Hope for Phin (you get the option to choose) no board members attacked me. Simply wait to give Phin the chems until Sophia tells you to go to Hope.
I killed the Roseway town after helping it for early cash/items. The entire location is optional, NOBODY cares. You DONT deal with Cleo past that.
I put Adelaide in charge of Edgewater so Sophia gave me more exp to kill them. Also nets a rare cheevo.
I helped the MSI on Monarch so I could get massive exp killing Grant and his Iron followers by NOT brokering peace (NOTE THAT PEACE MAKES TARTARUS EASIER).
Any easy or safe side quest, I did. I went back to old locations to do quests that I wasnt tough enough for earlier.
With 100 intimidate, the Tartarus comm guard gave me his key, letting me bypass most of the fighting there. For the mech boss, I'd circle the room, let the drones come to me, TTD and machine gun with electricity. Ran, rinse and repeat, The drones spawn every 25% or so of the bosses lost health.
Outside of these tips, I cant write much atm. All of our play styles differ and I went slow and sniper most of my game.
I will answer any and all questions, however.
Happy hunting (>*_*)>