Title pretty much says all you should need to know to figure this out, but I'll lay it out.
Working on late act 3 in Hell, but I'm sure this would also work in Inferno mode, I had Tyrael following me as a companion but just kept dying and couldn't take on the enemies I was facing, who kept throwing poison at me, going immune, etc.
I died and noticed that during my 30 second long revive timer that Tyreal stayed alive and kept killing shit for me. He's not the most powerful guy ever, but it's sure as hell better than corpse-running a thousand times.
I'm even getting the loot from his kills. Hell, he even completed an event for me while I was dead during the Siegebreaker quest. I'd just suggest going AFK while you wait.
I know many may frown upon doing this, because it is rather cheap, but I don't have many friends to play with so recruiting help was not an option. I believe resetting the game could have caused me to spawn with different enemies, perhaps no rare enemies with shitty affinities, but I prefer this. It also gave me a decent break from smashing my head on my desk.
Hope this helps anyone who's struggling in later acts! Good luck.
TL;DR: If you truly can't kill the enemies yourself, and Tyrael is with you, die and let him kill it.