I had to do this fight today on heroic with great players but an under geared healer. Our group build was a Paladin tanking, holy priest for heals, and then a rogue, mage, and hunter for damage and crowd control. So what we did was take every advantage of Vexallus's Health percentage per Pure Energy coming out with the group composition and it worked out well.
We started doing normal damage not having to worry about add speed because at the health levels:
85% -The rogue took the first set of Pure Energies and cloak of shadowed out of it, the one minute timer on cloak began.
70% -The hunter sent his pet in to take the next set of Pure Energy, we let it DPS until dead afterwards to save heals.
55% -The mage took the next set of Pure Energy activated all his racial, spec, and equipment cool downs and together the entire group tried to burn Vexallus as fast as possible.
40% -The mage took the next set of Pure Energy and ice blocked, immediately the group stopped DPS except for the paladin tank to build threat, we did this for two reasons, primarily we were waiting for the one minute cool down on cloak of shadows to reach its limit, and if possible the priest was to try and fix up the group as best possible. When the rogue asked for it we started DPS again.
25% -The Rogue took the next set of adds, cloaked and we resumed DPS our health being in decent shape by then.
10% -Vexallus began doing an Area of Effect and we burned him as hard as possible finally downing him, however the healer had to let a few of the DPS die to keep the other DPS, tank, and them self up.
Another thing I recommend is to use the macro
/target pure energy
and then either a /cast spell , /attack , or /petattack macro whichever you can do the fastest, In our situation even one Pure Energy AoE would have wiped us so we required the macro on the people who were soaking the Pure Energies.
Some other things we could have done differently are bandaging on the 40% stop DPS phase.
And also Major Arcane Protection Potion on the 10% AoE phase. 2000-4000 damage absorption divided amongst five people is 10k-20k HP the healer will not have to worry about though you may have to exclude the healer so he can use a mana potion during the fight.
Good luck and Happy Warcrafting :)
We started doing normal damage not having to worry about add speed because at the health levels:
85% -The rogue took the first set of Pure Energies and cloak of shadowed out of it, the one minute timer on cloak began.
70% -The hunter sent his pet in to take the next set of Pure Energy, we let it DPS until dead afterwards to save heals.
55% -The mage took the next set of Pure Energy activated all his racial, spec, and equipment cool downs and together the entire group tried to burn Vexallus as fast as possible.
40% -The mage took the next set of Pure Energy and ice blocked, immediately the group stopped DPS except for the paladin tank to build threat, we did this for two reasons, primarily we were waiting for the one minute cool down on cloak of shadows to reach its limit, and if possible the priest was to try and fix up the group as best possible. When the rogue asked for it we started DPS again.
25% -The Rogue took the next set of adds, cloaked and we resumed DPS our health being in decent shape by then.
10% -Vexallus began doing an Area of Effect and we burned him as hard as possible finally downing him, however the healer had to let a few of the DPS die to keep the other DPS, tank, and them self up.
Another thing I recommend is to use the macro
/target pure energy
and then either a /cast spell , /attack , or /petattack macro whichever you can do the fastest, In our situation even one Pure Energy AoE would have wiped us so we required the macro on the people who were soaking the Pure Energies.
Some other things we could have done differently are bandaging on the 40% stop DPS phase.
And also Major Arcane Protection Potion on the 10% AoE phase. 2000-4000 damage absorption divided amongst five people is 10k-20k HP the healer will not have to worry about though you may have to exclude the healer so he can use a mana potion during the fight.
Good luck and Happy Warcrafting :)