I think the problem with this card is this:
The logic of it is that it needs a high cost, because it is a creature and Quint in one package.
However, the one flaw in this logic is that you would never quint an immortal if it wasn't already immortal.
You would use quints on creatures with valuable abilities. So the cost should definitely be lowered to at most 5/5, or even increase the damage by one and keeping it as 6/6.