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.
The EASIEST possible way for killing the floaters is the homerun baseball bat that has near 100% crit attack everytime.