Krang et Shredder

If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the spell has any mandatory additional costs, those must be paid to cast the spell.

Some disappear abilities, such as those of the above cards, are triggered abilities with intervening if clauses. If a permanent hasn't left the battlefield under your control that turn by the time the disappear ability would trigger, it won't trigger at all.

You cast the card while Krang & Shredder's last ability is resolving and still on the stack. You can't wait to play it later in the turn. Timing restrictions based on its types are ignored.