Having read the latest comments, I must say I still have to side with Xavi. I see nothing undue with the concept of a MuCo(Aq) spell allowing for both toughness and the ability to move. If one wanted to get really technical about it, perhpas a Re req could be argued as necessary but I would then say that such a req would be free as it it is inherent to the intent of the spell that he become a functional creature of ice.
As for the further hardness of diamond spell, I suppose there is some validity to the critique regarding base individual size for gemstone. On the one hand this spell perhaps should be higher for additional size mags given its resulting protection (which I might even argue coveys a greater soak than +20 (after all go strike even simple rock with a sword and see how much damage you manage to do to the rock before breaking your sword). On the other hand, it might not even be necessary since a 12 meter tall giant of solid ice is already going to be hard as rock for anyone or anything that tries hitting it.
Perhaps instead, have a spell that simply re-forms the ice into any "wound" the giant sustains during a battle, like a ReAq(Co) spell of some sort (since ReAq can chnage water to ice or steam and thus vice versa).
Restoration of the icy wound
ReAq(Co) 15
R:Per D: Sun T:Ind
(base 3, +2 Sun, +1 size, +1 corpus requisite for cauterisation of wound upon reverting back to natural human form)
By means of this spell the magus is able to reconstitute areas of damage to his "flesh" whilst under the effect of MuCo(Aq). The ice surrounding the wounded area simply unfreezes and refreezes into the wound, offering a cauterisation benefit.
Upon reversion back to human form, wounds sustained whilst in ice form will appear as cauterised scars which would require additional proper CrCo magics to truly heal.
Something like that is cheaper on the fly and would perhaps negate the need for a spell for additional bodily transformation.
Just a thought.