I can't beat lieutenants. =/
I can only beat them with excessive cheese. Hanging just out of range and abusing their unwillingness to get close. I've beaten two of them like that so far.
I'm only level 1 though.
This has been my biggest complaint as well, is these fights. They just end up not actually being a "fight" at all, one way or another.
The only way to kill these guys is to cheese them or take advantage of AI bugs usually. The ONLY other thing I've found that works is using that fire spellset.... the one with Campfire.... and when the fight starts, leaping right at them, and then firing off the ammo spell so the entire thing hits them. Repeat a second time, and they're dead. No matter what though you're GOING to take some hits.
Though that, again, is not really much of a fight either.
Their attack patterns need re-doing, honestly.