Should their alter unpurgeable but only last 5s? That way its balance for all class with or without purge. Especially their Hunter rival class cant dispel their alter. Beside Alter only 25s cd with shifting power. Make their ice block undispellable too but only 1 block.
In current state of the game, where its a pure lottery if you purge it because there is 100billion buffs from your own class and your healer on top of that, i think its realy fine. Mages needs to have some weaknesses too you know.