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
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 17:01I made a video::