It depends on the aura. Its the same aura as given to uniques of the same level
Uniques have area level + 3 used to determine aura level, then the level of the aura is
blessed aim = level/5
might & holy fire = level/6
holy freeze = level/7
holy shock, fanat & conviction = level/8
So a level 99 mob can have 19 blessed aim, 16 might/holy fire, 14 holy freeze or 12 holy shock/fanat/conviction
Then you can add another +1 skills with battle command but I don't know if that will affect the aura already active, I assume it does not