We did this raid tonight, and although we didn't finish it I thought I would post my emote info in case it's helpful. FYI, four of this mob spawn (along with Nixk himself, although he doesn't aggro, just AE's) when you finally kill the Guardian: the Marauder spawns when the last of these four die.
Each Sentinel must be killed by a certain class. They tell me that it's a random class of those that are in the raid when the Guardian dies. Two different sentinels may have to be killed by the same class. The only way you know which class has to kill a sentinel is by killing it one time and getting an emote like this:
[Sun May 21 21:26:37 2006] Your skills were insufficient to defeat the sentinel! [Sun May 21 21:26:37 2006] A holy blade daunts the creature. [Sun May 21 21:26:37 2006] It is reforming to come at you again!
Here are the classes and which emotes indicate them (most gotten from The Steel Warrior website):
BRD.....Foreboding melody BST.....Deep gashes of feral savagery BER.....Shies from heavy blades CLR.....Dread of celestial spirit DRU.....? ENC.....Mind and body vulnerable MAG.....? MNK.....Focused Tranquility NEC.....Cowers from doom and death PAL.....A holy blade daunts the creature RNG.....True shot and fast blades ROG.....Ignores anything behind it SHD.....Dreads the strike of death SHM.....Appearance of talismans WAR.....Brute force and brawn WIZ.....?
Note that riposte and damage shield kills count as the class that riposted or had the damage shield. So make sure your tank doesn't have DS on when the mob gets near death.
MAG.....Summoned elements WIZ.....Power of elements
Mage pets do not count as a mage killing the Sentinel, so make sure your mages get their pets off as well if it needs to be killed by a mage. Last night we had one Sentinel that had to be killed by a mage, a mage-pet got the kill shot, and the sentinel respawned.
Once you kill one of these things, theres an emote which gives you a clue what class needs to get the killing blow to finally kill it. If the wrong class gets the killshot it rivives with 100% HP.