I've had Lech Kril get stuck in the Phobos fight between the Ice and Fire phase a few times -- he crouches down and gets surrounded by a fire "bubble" and is unhittable. I discovered that a Dark Dagger proc will knock him out of it for awhile -- he'll fight normally for a bit, then go back to the "bubble" and need to be proc'ed again to knock him out of it. Not sure if any other weapons will work but I've had the Dark Dagger work 3 times now.
Guys, I've tested it and the bubble should only appear when you're unhittable according to his AI. If you can't be reached but have Line-of-Sight to him, he'll bubble so you can't be cheap and fight him from afar.
Somehow, it must've messed up and detected you as untouchable even at close range. Which is weird, since he won't do that as long as he can at least hit you with his hammer throw.