This may have been patched. The guild I'm in is very strict about not using any exploits of any kinds, but as of last patch it was a working exploit. (Nor do they really do AQ20 anymore anyways..)
Would someone please check on it next time they go to AQ20 and kill Ossi?
All this requires is a hunter of some sort, and 17-20 people that somewhat know what they're doing. AQ20 isn't hard, although many guilds have troubles with it. You must be able to clear the other bosses excluding Moam to get to Ossi.
First, have your raid back up. Far. Have a hunter stay as far away as he can, or far enough so he can use eyes of the beast and still aggro. Send the pet in, aggro, break eyes of the beast and Feign Death DIRECTLY after. If you did it right, a crystal will spawn - and stay up.
How the Ossi fight works is basicly a huge kite. Two/three tanks take turns dragging him to crystals, and as most guilds know, most wipes come from bad luck. Every (minute, is it?) Ossi will enter a "god mode" killing everything in his path, unless you use a crystal to stop him. After a crystal is used, another will pop up.. Sometimes far, sometimes close. This can be a problem, because your tanks have to keep aggro and your healers have to move while getting both pinted and slowed.
Anyways, down to the catch. If you use this method several times, crystals will keep respawning and not go away, which means you don't have to kite him, and you can keep DPSing without any trouble!
GL, hope it still works. It will definetly help Ossi become quick and easy for epics!