Corrosive, they're the ones, the big green bastards !
Oh yeah, I'm level 18, been playing it splitscreen co-op with my brother (Waiting for my mate to hurry his arse up and get it so we can play together !)
Me and my brother are both Soldiers, I'm more DPS orientated with the Infantry skill tree, while he's gone a little in Support and Medic, since he loves slapping the turret down whenever we find a bunch of Badass enemies (They're always in groups of like 3/4 !!!) and it gives us ammo.
I use my turret... when I remember, and I've got an artifact that adds Incendiary damage to my turret, which also helps with the DPS, but to be honest, I hardly ever use it, lol.
So far, the only real problems we've had, were getting the cave key from Sledge's hideout (Haven't actually gone to fight him yet, how far through the game is he by the way, cause we've been playing for around 8 hours and feel like we haven't gotten anywhere), because that really big psycho was just... ugh, how the fuck does he live for so long ?! And then the one where you need to kill Moe and Marley, well, Moe was hard as fuck since he runs faster than you can sprint away, and he takes a real beating, and bullets don't slow him down either... AND there's the fact that there's tons of Badass Skags around, which just make it worse when your shield is down and they're dealing damage faster than your health packs can heal it... And as for Marley, well, she (I assume it's a she), was pretty easy, when she shoots her green ball, jump out the way, and maintain your range otherwise she runs off, and ends up coming back with more...
So yeah, as you may (Or may not, but if so you're stupid) have noticed I'm REALLY liking the game.