High On Life

Can be easily boosted right after you get the B.A.L.L. weapon. Pick the B.A.L.L. gun and run past the guy, who gave it to you. You will see a gaiser that can be opened by shooting two holes with the B.A.L.L. gun next to it. Shoot them to fly above the ground to a small "dungeon". There are going to be at least 2-3 waves of Poacher ants.
Pick the Harpers gun and shoot then with button. You can wait for a cooldown to reacharge the gun or just press and press "Last checkpoint". You will start right in front of "closed" gaiser. Repeat and you have 50 enemies killed.

An easy spot to obtain this after finishing the DLC is on the same island as where Toxxo is located (Bather’s Isle).

Once you arrive at Bather’s Isle there will be a small rock formation with two arches. You can use the glob shot to take out six of the small crab looking creatures (apologies I have forgotten their name).

Once you have killed them, head north towards the rock formation with the pink slime trailing down it. To the left of this is a small enclosed rocky area where a poacher is standing on a rock fending off seven more of these crabs. Again, use the glob shot and take them out. This is fairly easy to do in one shot as they are bunched together.

Once done, press start and load last checkpoint. This will load you at the start of Bather’s Isle. Rinse and repeat this process until you obtain 50 kills.