which ones?...the ones with the question marks, yes....the others, no....some just take a lil longer to kill than others, and you have to shoot them in the right spots...
so far, the only enemy I've struggled with killing is when it's a public event and you have to stop the enemy from reaching checkpoints...those dudes have a shytload of shields to get through...and they fast....if you don't use the right weapon effect to get through the shields (think void works best on them), it'll seem like you not doing any damage to them no matter how much you shoot them...
Yeah they have ???. I've encountered a lot of them. There's some dudes down some dark stairs on the first level. They stay there if you come back