Solod this as RDM/NIN earlier today. Both Phanduron and Drexerion were up, so safely pulling phanduron was a little tricky.
Basically, after buffing up, I threw gravity at Drexerion from afar. This linked Phanduron (they tend to stick close together all the time..), which caused him to start casting something (blizzaga 3). I back out of the room a bit, reapply sneak, and start running back. I ran all the way to the ronfaure zone, which is quite a run away. To make things longer, Phanduron would frequently stop to cast something, though I was always too far to get hit with it (often times I was too far to even see what he was casting!), which made the run excessively longer than it had to be (about 10-15 minutes), but made the run all the easier. If sneak was to wear off, I could safely recast it as long as I keep my distance from him. Also, despite never actually performing a specific action on him, I never lost hate on Phanduron; he chased me the whole time. I usually ran just out of targeting range when he stopped to cast a spell, but maybe he would have still chased me if I ran to the zone without stopping.
I never saw Drexerion on my run to the zone, so it's possible I lost hate, but just to be safe, I let Phanduron catch up to me a bit and zoned. The small hallways with werebats near the zone is a good place to kite it; no aggro and there's a decent amount of running space. You'd have to be careful of blood aggro, though. I myself melee'd him at the zone. It's a lot safer, IMO, since you can simply throw a DoT on him and zone for a little bit if things get a bit out of hand.
As for the actual fight, he isn't terribly difficult, but he can be very dangerous. His regular swings take longer than that of a normal monster (each swing had a delay about the same as a great sword), so with slow II his swings were very slow, and keeping utsusemi up was no problem. His regular hits hit for about 70-80 damage from what I saw. The hardest part, though, is his black magic (and even worse, manafont). With a hefty amount of MND (about 100 total) his tier 3 -gas only did around 300-400 damage, with the somewhat frequent resist (wouldn't even break through a weakened stoneskin) and the rare unresist (probably 600+ damage; would eat through a fresh stoneskin). He casts a spell every 20-30 seconds or so, and doesnt seem stunnable (I tried stunning firaga 3 with flat blade, but stun didn't go off. I only once, so maybe he lucked out).
This monster isn't as unintelligent as you'd think. At one point in the fight, he landed a sleep I on me, and followed it immediately with manafont, which he followed up with a tier 3 -ga. He also tends to cast one or two weaker nukes or spells (such as blind) before throwing another -ga at you while manafont is up. Due to this, he only got off about 4 tier 3 -gas, which isn't too bad, though I was still ready to DoT it and zone until manafont wore off.
Also, a tip to others who are going after this NM (again, that is); if you did not kill Drexerion with Phanduron, wait at least 12 hours (preferably 24 hours or a bit more, depending on how inconvenient Phanduron's ToD is for your schedule), then go back and kill Drexerion. This will save you the trouble of having to zone Phanduron again (though taking him to the zone makes the fight that much easier if sh*t happens, so it's really up to you).
Finally, good luck to everyone! Currently 0/1 :|