I'm noob, this is no pro guide, just showing very basic tactics since there seems to be too many people having problems even with one of those.
They have shield.. use electric ammo whenever possible. If you don't have electric, use NONE. DON'T use fire or corrosive at least.
Make sure you hit them with at least one bullet every 5s, when when running away or shooting snipers..
1) basic tactics - HIDE (all classes)
this is plain raw fight. No skills. Used SMG with lowest dmg/ammo.
https://www.youtube.com/watch?v=Vjujex4GrUE
2) range/sniper - they will miss (all classes)
at first half of video there is safe distance they will miss rockets and cannon attacks. Just go left/right when they shoot. It's usable outside, especially when you (or them) can't hide.
https://www.youtube.com/watch?v=kxZQ1LbexT4
3) use your class and tools
every class has something to help. This is where it goes if use class skills (assault in this case), perks and pulse grenade
https://www.youtube.com/watch?v=KRE5_xyw3lM

