The Bug Butcher

A great place to grind for this achievement is mission #11. At the very beginning of the level the Gatling Gun will spawn every time twice in a row. Also, there's tons of small enemies to rack up the count quickly. So grab the power up the two times and kill as many enemies possible, then just hit retry. Got it very quickly doing this.

Arcade mode - Floor 2 - Level 10 is the best level to grind weapons after you beat the entire Arcade mode and upgrated everything.

In this level there are those blue enemies that spit pink aliens, do not kill them and let the map be full of enemies, the game will start spawning different type of weapon more frequently because with so many enemies you will get a lot of combo and power up.
When you collect the weapon you need, do not fire fast, but try to dose the shots in order to make the weapon last longer and hit more enemies at the same time.

I got 4 achievement in a couple of hours:
- BULLET HELL gatling gun You won this on 21 September 18 at 16:19
- BEAM MASTER laser beam You won this on 21 September 18 at 16:36
- EXPLOSION EXPERT rocket launcher You won this on 21 September 18 at 16:57
- ELECTRIC WONDER chain lightning gun You won this on 21 September 18 at 17:01

I made a video: