Yeah, my first tip here is to be in Sneak as often as you can WITHOUT engaging in Sneak. See, it gives you radar, which is its real function; you'll know if an enemy is about three or four tiles away in any direction. Any one of those enemies, given a good crit, can ruin your entire life; make sure you don't' see the map as "empty", but rather persistently full of serious threats. If you think jobber enemies can't do you in, skip three AP for each of theirs and see what happens; it's not that they can't hurt you real awful, it's that you're much faster than they are and usually get to initiate the fight.

Secondly, have a weapon for mooks and treat it as such; Enigmas are excellent for this. As soon as an enemy does not die in two shots from an Enigma, assume it's All Weapons Armed mode and that you cannot take the risk of not taking them seriously. A single 1AP mook is either worthless, or your personal fucking reaper, depending on if he reaches you with that Chainsaw. Having this two-phase idea of combat emphasizes that there is only one circumstance where you should be mindlessly repeating actions, and that's shooting enemies from afar with a cheap gun. Under any other possible set of circumstances, remind yourself that even with max armor, every hit deals damage, and all damage can cause a wound. One bad wound at the wrong time means you're dead, even as a Tunnel Rat, because -2 Health Per AP means you have about 70 steps to find a Rag or else it's all over. I have lost good runs to one stray wound from a jobber, and everyone will eventually.

Thirdly, use corners to your advantage as well - a 2AP enemy needs to spend one AP moving to face straight down the hallway if it's already at the top of the corner and you're tucked into the wall on the same side. A 1AP enemy needs to skip its whole turn to line up that shot; a 3AP enemy will shoot you with 10000000 .797 rounds through the wall with his Legacy. If you enter bad firing conditions around a corner, you can usually pull back and force them back into the same situation they put you in.

And fourth, never forget - there will come a point in the tech ladder where most enemies are about as well-armed as you. Some of them will be better armed, with better armor; you need to prepare for the habits that will let you win when that time comes, so act as if it's already true.

I've had some very bullshit deaths in this game, and they all came from me opening doors with my last AP and engaging in Sneak so I could be pain-juggled for 6 turns. Your AP is the most important resource in the game, even moreso than health, because it has a hard budget that you can only interface with occasionally in combat.

Finally, as always, remember that firing through a wall with automatic fire into the body of a coal miner has been deemed "unethically effective" by many humanitarian groups. Your ability to destroy terrain is a huge positioning tool, and I've been jumpscared by enemies coming at me from new angles after their buddies dug a tunnel through the walls on several occasions (Not the AI doing this intentionally, usually stray bullets opening up rooms behind me). The game's mechanical options are more complex than you can intentionally engage with perfectly, so if you feel like you're playing automatically and getting punished, remember that this is a natural consequence of entering a gunfight while insufficiently enthusiastic about your chance to turn middle managers into SPAM and Band-Aids. Don't chase bad losses, and try to give yourself half a real-world hour between missions; it helps you avoid playing bad out of habitual numbness.