Question: I'm having mega trouble killing them: I've tried bombs, shotguns, etc. but they're still a nuisance!

Are there any tips or tricks to taking them out?

Answer: 
I found that melee weapons work the best. Charge in, circle strafe around it and swing away with a melee weapon.

Or if you are able to knock them over, stomping on its head will also do it.
2 grenades or deobombs do the trick for me, especially when in a cramped space...
i always stock up and, of course, carry (lol) incendiary weaponry that i use against floaters specifically