Great game, but I have a small remark:
The firebolt deals extra damage to slimes and such and it costs 25 sigil points. Currently on average I get 115 damage on a slime.
Twin Strike costs only 20 sigil points and deals 125 damage on average against the same slime.
Even tough Firebolt is more expensive, it deals less damage, even when the bonus is applied. This seems weird, as there is no reason to use this attack.
I would recommend either lowering the segil cost to 15, increasing the bonus damage, or add an additional effect to the attack (such as burn).
I would advise the additional effect, as this would give players a reason to use this attack for other enemies aswell.