Fist of Suns

If you pay {W}{U}{B}{R}{G} rather than pay the mana cost of a spell with {X} in its mana cost, the only legal choice for X is 0.

If you apply Fist of Suns's alternative cost to a spell with {X} in its mana cost, X is 0.

Fist of Suns's ability is an alternative cost to cast a spell. You can't combine this with other alternative costs, such as flashback. You can pay additional costs, such as kicker, in addition to this alternative cost.