text: You cast the spell while The Dawning Archaic's last ability is resolving and still on the stack. You can't wait to cast it later in the turn. Timing restrictions based on the card's types are ignored. 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, you must pay those to cast the spell.