***SPOILER***
*
*
*
**
*
*
*
*
*
*
*
*
*
**
You only turn to evil if you turn into the Slayer when Saverok prompts you to. If you turn into the Slayer to fight Irenicus at the end, then it doesn't have any effect upon your alignment. The final battle is not part of your "tests".
However, Reeka, if you intend to take your final save and replay through to ToB when it comes out...I wouldn't get too comfortable with the "turn into the Slayer" tactic. If you do, then you might not be able to beat him on right before your progression into ToB w/out using the Slayer form. If you're not worried about losing 2 rep. points, however...don't worry about it. From a role-playing perspective...if your monk is LG, I'd not change when you play through to ToB.
Tactics:
I ended up pulling a "Monty Python and the Holy Grail". Run away, run away, run away!!!
I took my party down into one of the tunnels positioned just in front of a staircase going down. I then proceeded to summon Skeletal Warriors and Elementals. I then cast Haste, Bless, Chant, Prot. from Evil 10', etc. and took my PC up to the door to open it for Irenicus. After he opened it and blew me backwards, I popped up in front of him again (the rest of my party remained in the tunnel where I had left them). After his little diatribe, I hauled my ass back to where I left my party, equipped them all with their missile weapons, moved my Skeletal warriors and Elementals to the front, and waited for the demons. When they came, they could only get at me 2 at a time. Between the summoned monsters (and my mage summoning more for backfill as they were destroyed) and my incessant pummelling with missile weapons and spells, they were toast by the time my last summoned creature was destroyed. I then sent my PC (Inquisitor), Minsc, and Jahiera up close to Irenicus to kick his butt. As they approached Aerie, Nalia, and Imoen all cast Breach. Then Imoen cast Khelben's warding whip and Nalia cast Lower Resistance, while Aerie cast Ruby Ray of Reversal. The three tanks then proceeded to whomp Irenicus/Slayer.
Whew! Now I'm tired!
BTW...don't waste your time summoning Nishruu/Hakeshar or other creatures to fight Irenicus. He just casts Death Spell and dispels them all...
------------------
Now where did I leave that doughnut?!