BattleParty

Buy the gold upgrade dlc to unlock the Tank from the store to save many hours of playtime (~7 hrs to 100% with DLC and 10-15hrs without):

https://store.steampowered.com/app/1290380/BattleParty__Gold_Upgrade/

Use tank to get fast kills and stay invulnerable (Vehicle cannot be destroyed)

Walking over and staying inside the confines of a car vicinity will also make you invulnerable to enemy fire.

The game currently suffers from fps instability due to bad enemy spawn scripting. Play in windowed mode, on very low graphic settings to postpone framedrops for 5-10minutes. Kills are gained cumulatively so you can just go back to the menu and start a new round, whenever the frames drop too low.