Defeating Your Enemies
The beginning tool works to kill an enemy. It just takes quite a bit of time – check if you’re using the correct button, as you have build and dismantle rays, only one of them may be able to hurt the mobs.
You can also run them over with a vehicle.
Another thing you may want to know is that they will despawn (going far enough) and if you have destroyed their nest, they will not respawn, so if you have nests too close to your current base / outpost and you keep having to deal with the mobs, make sure you destroy the nest, then you can just wander away and come back, the area will be cleared of mobs without you killing any.
Note: There are rifles you can build as you progress, but they’re not strictly necessary, you can deal with enemies in a number of ways without ever using a rifle.