They can be a right pain, unlike almost all other opponents they can not be steamrolled. You need to take out any little minions, then kite them basically. I have had a number of deaths were I could not stand back up and run away after. Spamming jump pack was the only way to get out of the cycle. I now have better sentinel armour and the turret helps with a distraction.

I would like to see some game footage of people having this problem. Purely because I find them annoying but not the point where they are unbeatable. I wonder if its also influenced by equipment or play-style choices. 

What weapons are you running with? I run with a 2h hammer and 2 bolt pistols and I've not encountered anything that I would call difficult. My guys is only 20lvl R 6, so maybe you're encountering something farther along the way? Try getting a couple of pistols for weapon swap and going in with the sentry gun armor. Then, it's just a question of dropping sentry guns and running away/ in circles. If you want to use the a melee weapon, use a 2 hander and  jump armor, just jump in on them, stun, smack them around, jump out, run around until jump recharges, jump back on them repeat until dead. If close in isn't your thing, have you tried doing some aimed shots with a las rifle? How about dropping a bunch of plasma cannon mortar rounds on their head. It's really hard to help if I don't know what weapons, armor, and skillset you're using. I will try to help more if you can be specific about your setup.

any SM dies after 3-4 shots from Omnissiah blessed gravigun