The Bug Butcher

GENERAL STRATEGY FOR ALL KILLS ACHIEVEMENTS:

I found Level 28 (Observatory) to be a great place to boost your weapons kills.

At the very start of the level, run all the way to the left side of the screen. You can stand here for a few moments without any enemies hitting you. Once the words "Wave 2" flash on the screen, run to the center of the stage (you will get hit once at this point). In the center, you will be clear of enemies again (due to the force fields).

At this point, one of the powerup weapons will have appeared (or will very shortly). Pick it up and start getting your kills. Don't worry about getting hit at this point. Kills count regardless if you start over or die.

Note:
-Upgrading weapons using coins will help getting kills.
-Weapon powerups are based on shots fired, not time. So you won't run out of a weapon until you fire off all the shots (meter in the bottom left corner) or pick up a new weapon. Don't feel like you need to fire off all your rounds immediately.
-Weapon powerup spawns are based on timed intervals. Even if you are holding a weapon, new ones will continue to spawn.

ROCKET LAUNCHER SPECIFIC:

Each time you press the 'A' button, a cluster of rockets will deploy. Try not to hold down the A button as you will quickly fire off all your rockets. It is best to stand under a group of enemies and just tap once.

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::