Maybe this is how most ppl, my whole party is around 11th level with my main Ranger who is 13 and has the moondog...
ANyway I got Jan to cast 6 traps this killed his first form then I left the room went outside and rested!
THen I used 1 protection from magic scroll on my ranger made 5 of the rest of my characters invisible went into the room. My ranger went right up to him used Flame tongue +4 vrs undead to attack him, the rest of my party went of into the corner to hide. Well my lvl 13 ranger using flametongue and that scroll killed him easy as pie.

All you need is a +4 or better weapon ONE character basically any LEVEL and protection from magic as long as the rest of your party is out of the way!

OR If your a bounty hunter you could set a maze trap with normal traps, once first form dies, place MORE traps while he's mazed, and repeat the process!