Stalker

These guys are from Borderlands 2 & The Pre Sequel They have a gland that works as a shield as well as a cloaking device that turns them invisible until right before they attack you. Now that would be enough to irritate the piss out of anyone right? Well that's not all, they are also fast as shit so when they do turn invisible all you are able to do is keep an eye on your mini map and move around until they become visible again. They also tend to travel in packs.

The good news is that once their shield is all the way down they can't cloak anymore either so all you have to I chase them until they're deed. The best way I've found to do tho is with a long range weapon (like a sniper rifle) and pick them off from afar. An Elemental weapon also does good if you have one. Shock weapons are the best choice because the shock damage saps their shield real quick.


Giant Spore

These guys are also from Borderlands 2 and are my honorable mention. Not only do they float in the air making it harder to kill they come with 1 of 5 elemental types meaning they do even more damage when a shot connects. As if this isn't bad enough they pop out little spores that explode on contact and also do elemental damage of the same type as Mama spore.

They do have some upsides though. They follow you and if you do it right you can bring them to another bad guy and they'll attack each other, making it easier to kill them both.

However when you do kill it it explodes into a handful of the tiny spored that you then have to dodge until they touch down and explode