I took my test last night. The first part at lvl 18 was a piece of cake, kill some stuff in Wailing Caves. The second part is something you have to really know your job for. Took me 2 tries, the first try I just went in and started nuking...and found out too late my mistake and had to bail. So I made a battle plan, one that worked pretty good.
So, for all you evil Clerics about to go Inquisitor, here's some hints on how to do it:
Make sure beforehand your Radiance is ready to use.
You will go into a small room. There's some gnome named something Dim. Dim will rant for a moment, and then he'll summon two undead that are standing by the door. Go towards the door, start the fight.
Now, don't nuke like mad as you normally would if solo'ing. Conserve as much Energy as you can. If you have skeleton Master Smite, do a piety chain with this whenever it resets for casting. Toss in a nuke here and there, but really you have to wittle these down by melee. They don't hit really hard, but if you have to...cast a bestowal of Vitae to keep your health good.
Once you got those two, stand near the door. Don't run back towards Dim. He'll go on a rant again, and summon two more undead, but these are off in the far corner. If you move too close, they attack. But if you stand near to the door... you'll have time to recover full health and energy. Once you're ready, pull them over with a spell, but don't go towards Dim. This is very important.
Wittle these two down in much the same way.... melee...toss in a nuke here and there...but try as much as possible to keep your energy up. The last set of undead Dim summons will run right towards you no matter where you are. BE SURE TO KEEP BACK FROM DIM! If you get anywhere near him, he'll start to attack too, and at this point it's very very bad if he does. You also can't attack him at this point, because it will void the quest and you'll have to start it all over.
Beat the undead, same as before. Tedious and long fights, but beats running out of MP, because next you have to fight Dim. Cast Radiance on yourself right away...this takes no Energy to cast and you'll need it. Dim casts a spell that will make you unable to use your own spells, then he'll start super nuking you. That's where Radiance comes in handy ;) let him blast through his MP, and once he's done...you'll probably need to heal yourself once the No Casting spell is off. After that, he hits pretty weak, though he does have a spider with him that can do "meh" damage. Just focus on Dim, however and nuke the heck out of him. Once he's dead, the spider's dead too since it's a summon. Viola' You're an Inquisitor...well, will be once you hit 20. You get the Inquisitor's Hammer @_@ which is sweet...
It's definately a drawn out fight, took me over 10 minutes, but it's a pretty solid way of beating him. Explained this to my friend, and he beat him first try... so give it a shot ^^ and good luck.