I do 15/15 Crusade with Admiral, Death Mage, Vengeance Spirit, Shadow Shaman and Frost Mage ( i don't use Death Knight anymore) and have been doing so for many many levels

(I am lvl 86 now)
I always have full power at the start for each fight, so whatever team i'm facing, the first thing that happens is that Death Mage runs her ghosts, Shaman heals Admiral (while she is stunned by enemy Commando for example), and than Vengeance petrifies most of the frontline. Arcane Sapper will always be petrified for example, and then dies quickly to Frost Mages Ice bomb.
By this time, Admiral now has been healed for a bit, and now got the Armor Boost from Frost Mage as well as the Shield from Shadow Shaman, and then we just push through with Death Mage Ghosts, Vengeance Spirit Arrows, Frost Mage Ice Bombs and Admirals Blade.
Have not had any problems whatsoever with any team composition ever since lvl 60.