Going to reiterate what others have said above for 25 - go destro with mannoroth's fury. Have at least one ember going down, RoF on one or two of the bigger clumps, target a spirit close to the center, FnB immolate - FnB incinerate and continue to spam FnB incinerate on the closest ones to the center you can get. You're literally hitting almost every spirit with it and generating insane amounts of embers while doing so. You'll have 2-4 embers coming up, depending on how many spirits there were, to dump on the boss. It's an absolute joke how overpowered destro is for 25 man totems.