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.
Fire won't work vs. Floaters because they are almost always standing in water. 
Poison doesn't seem to work (even with Plaguebearer), probably because that is the Floater's base elemental dmg.
**Shock modded weapons** seem to work the best of the elemental damage mods, and can be applied with a pistol or rifle at a safe distance. A Tesla Bomb knife might be able to do the trick, but I haven't tried in the higher levels, so I'm unsure if it is automatic --> one good possibility.
**Impact mods/weapons** (Shocking Crowbar, Heavy Duty Kanabo, Left Hand of Glova, or the Homerun Baseball Bat (developer's mod)) can be thrown/used and knock him down and then you can stomp on his head. --> this is nearly automatic, as long as you can get him down, you can rush in and stomp.
**Explosives** (Sticky Bomb knife, Deo-bomb, & Grenade) - These do the trick nicely, you just need to be careful in tight spaces like the Sewers of Moresby so that you don't hurt yourself in the process. 
**Vehicles** care free and easy, you can either kill them or avoid them, your choice.

As for the ***best***, it would be situational;
 - If you're in a confined space, try the impact+stomp route.
 - If you are at mid-range and have room to move, use a grenade/sticky bomb or two.
 - If you've got line of sight and distance, try a rifle/pistol with a shock/pride mod and just keep plugging away at him.
 - If you've got access to a vehicle, then by all means run him down (or go around him).