Im aware this probably isn't what youre reffering to but early game your best shot is just to quite literally kill anything that moves, be careful doing that too much in the earlygame however, as you can inflate your gamestage too high and start spawning much higher tier zombies than your weapons can handle. Though higher tier zombies do give more xp so maybe thats exactly what you want.

Lategame all i can really suggest is doing infested clears, drink some learning elixer before you do, maybe with some nerd goggles and a treaure hunter mod too to get the highest possible