Well, did the Ony 10-man run yesterday, they made the bugger too eesy...
Range DPS, stay on Onyxia when little welps are comming, no rush, let the tank get aggro on little buggers. Give it a good 10 seconds, then AoE the welps, just one or 2 casts and the welps are dead, continue straight away on boss in the air...
Nuke boss till she comes down, let the occasion little welps be picked up by offtank, take them down. Continue on boss, boss killed.
She did only one Breath of Fire when she was in the air, since nobody got hit by it, we gained the achievement for that.
Quicky 2 minute fight.