I try to cast Shatterskull Snashing for X=6, but I can't spend mana after choose target creatures. (I'm sorry if my english isn't well...)
If you select X as a value that would make the spell cost more than the total mana you have the game does nothing. Expected behavior is that it says “You do not have that much mana” and present the auto pay adjusted screen. “x” costing spells should have an auto pay option to spend all mana to maximize X
and your English was perfect
I think it only wants to cast equal to the toughness of the target creatures?