Tikhar locations:
Can be found during Lost Tunnels, near the beginning.
Can be found during Dead City 1, in the ranger cache.
Can be found during Dead City 2, in a dumpster.
Can be found in a broken train car during Frontline.
Can be found in Defense.
Can be found during Black Station, in several wooden caches.
Find a Tikhar early and start killing enemies with it. Make sure to purchase a lot of ball bearing ammo.
Kill methods:
1.) Use headshots (fastest method). For guys with helmets, one shot should pop them off, and another should be enough to kill them (2 shots total).
2.) Kill weakly armored enemies (2-3 shots each). Guaranteed head shot kills.
3.) Kill nosalises. They go down in 4-5 shots and are found throughout the game.
4.) Amoebas. You'll encounter a lot of amoebas near the end of the game, especially when you go to power up the reactor.
If you start early, you'll probably have this within a few levels. If not, then kill all of the amoebas at the end of the game.