*If you do not feel like reading my guide just skip to the bottom.*
Each time you kill an enemy, the difficulty of the game increases slightly.
The game starts at difficulty 60 where the only enemies that should be spawning are the fighters and the boats.
Once you reach difficulty 160, jets start spawning.
At difficulty 220, battleships start spawning.
At difficulty 340, aces start spawning.
At difficulty 480, submarines start spawning.
Finally at difficulty 600, the blimp spawns.
You might ask: How do you increase the difficulty?
Killing a fighter increases difficulty by 15
Killing a jet increases difficulty by 25
Killing an ace increases difficulty by 80
Killing a laser ace (has a 0.016667 chance to spawn rather than a normal ace) also increases difficulty by 80
Killing a boat increases difficulty by 40
Killing a battleship increases difficulty by 160
Killing a submarine increases difficulty by 250
Killing an acemarine ( has a 0.3333333333 chance to spawn rather than a normal submarine) increases difficulty by 1500
Killing a missle (spawned by blimps and submarines) increase difficulty by 5
Killing the blimp increases difficulty by 1000
*Your Multiplier does NOT change the amount of difficulty each enemy gives*
You also passively gain difficulty by second (presumably 1.11 difficulty per second)
Thank you for reading my guide and I hope it helped you!
(This information can be found in OS (C:) -> Program Files (x86) -> Steam -> SteamApps -> common -> Luftrausers -> data -> res -> enemies & spawncoordinator [it might be different for some people with different computers] )