When you pull Ambassador Grindstone (Elemental Avalanche and Powertap AoEs) he will call a bull to his side by the name of Kanogan (Elemental Stampede AoE). You will want to tank these apart or they will become more powerful. However not too far apart because Kanogan can mem wipe and there are adds to kill.
Once the ecounter starts only Grindstone will be taking damage so all dps on him until he reaches 20% at which point he will be immune and you turn your attention to Kanogan. Once Kanogan reaches 20% Grindstone will go through a small script and disappear leaving you the chest.
One minute into the fight and about every 70 seconds six adds will spawn with the name 'a geonid warrior'. Burn these little dudes down.
Kanogan has a curse that is cast about every 20 seconds called Bullseye. You have 4 seconds to cure it before Kanogan mem wipes and does massive damage.
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Challange Mode
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You will need two tanks on Grindstone. He has a death touch/mem wipe curse called Touch of Stone cast every 55 seconds on his current target. This curse doesn't have to be cured but if you have enough healers to cure all the Bullseyes and this I would recommend it.
Curing curses will prevent the bull from memwiping and makes your tanks' lives a lot easier.
No frontal on the bull, can stack up right by it, just joust on red text.