Materials test your tactics.

• Fill all your slots - You want to protect your core. Any damage that goes to one of your parts is damage spared from your core. There is no reason to have unused slots other than not having enough items (usually due to recently losing your items) or having not enough resources to equip items (10 matter and 20 energy for equipping an item).
  
  
• Always carry backups - Your parts are more fragile than your core, so you will lose them often. It is crucial to replace your parts (i.e. weapons), even in the middle of combat, to regain full functionality. Running out of items of a type often spells doom for the entire run (i.e. losing all weapons makes you defenseless AND you can't hunt hostiles to get a weapon; losing all propulsion forces you to drop most of your stuff).
  
  
• Replace items during combat - Replacing parts under enemy fire sucks, but it is worth it! You should usually take your time to put on new items even if it's just for their coverage. As long as you can replace your parts faster than they get destroyed, you'll be fine. Of course it's better if you can duck behind a corner before spending multiple turns on this, but that is not always an option.
  
  
• Reposition frequently - Complex has the numbers advantage if you fight in the open. Nullify it by going back to a choke point. If you get flanked, then reposition again. It is usually worth it to take a few shots while running for cover in order to be able to fight enemies one-by-one. If you want to do something else than firing weapons for a few turns (replace destroyed parts, wait until your temperature goes down, etc.) you can take fewer hits overall if you just run for cover first. Remember that a few shots can't destroy you.
  
  
• Doorway tactic - The most common chokepoints are the doorways. Stand diagonally adjacent to a doorway (as shown in the picture below) during combat. This forces enemies to fight you one-by-one. The enemy who stands in a door will block his friends and no other hostile will be able to take a clear shot at you. Beware of other entrances to a room!
  
  
  
• Safe exploration/movement - keep close to choke points and stay away from explosive machinery, so that when a squad spots you, you can deal with it with minimal damage received. When a squad spots you, run to a choke point in an ALREADY EXPLORED area. You know that there are no more hostiles or explosive machines there. There's nothing worse than running away from a squad only to be spotted by another squad - it's a potentially run-ending situation.
  
  
• Dealing with dodge-reliant enemies - Some enemies (such as Pests) heavily rely on their dodge chance. You can completely nullify this advantage by using a launcher - they can't dodge explosions. A launcher shot will explode when it hits a target (hard when dealing with high dodge chances) or hits a wall (make sure such enemies stand next to a wall and target the wall instead) or when it reaches it's full range (keep enemies around launcher's max range). Second way of dealing with such enemies is to use multishot weapons, such as shotguns and flak guns. More shots fired equals more chances to hit the target. Lastly, you can counter dodge chance by gaining accuracy (via targeting computers, close range, standing still, etc.). Bonuses to accuracy are more important when chance to hit is low (gaining 10% accuracy when already at 80% is barely noticeable, but gaining 10% when at only 10% doubles your damage output!).
  
  
• Prefer high damage weapons - Most enemies in the early levels have 15 or 20 core integrity. A weapon that deals 20 damage is guaranteed to one-shot such enemy if it hits its core. Surprisingly, a single shot of 20 damage is way more effective at this than 2 shots of 10 damage each. Multiple shots tend to spread damage across components and leave all of them functional, while larger chunks of damage tend to destroy the few components they hit.