Go into your skills and map the Dark Magic skill: "Insidious Doubts" to a button.
When you first get to the Lozrun King just kill his minions and then hold R to auto target him, and he'll forever recycle through healing himself and then immediately attacking you with a ranged attack after that. This never changes.
So just stand next to him with auto target locked on and move to the side just a little bit right after he heals himself, as to avoid the ranged attack that immediately follows. Wait until his power gauge(the blue bar) completely depletes, and then start spamming the Insidious Doubts skill every 4 or so seconds(depending on how much you leveled it up). The skill drains his power and keeps it at zero, and you know you need to spam it again the second you see his blue bar start to gain again. Hit him with whatever deals damage in between until he dies.
I was playing a techno melee class and only had to bring Insidious Doubts up to level 1 to kill him, so it's really easy.
This seemed to be the easiest way to kill him to me, but I know quite a few people get to him and he seems impossible at first, so here's your easy way to kill him.
GameCube Defense Force - PSN: Gigamog Playing: Sonic Unleashed