Narset, Enlightened Exile

Narset's first ability gives it prowess while it's on the battlefield.

If you cast a spell "without paying its mana cost," you can't pay any alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, you must pay those.

If the spell you cast has {X} in its mana cost, you must choose 0 as the value of X.

If you don't want to cast the copy, you can choose not to; the copy ceases to exist the next time state-based actions are performed.

If a creature has multiple instances of prowess, each triggers separately.

Once a prowess ability triggers, causing the creature to lose prowess by removing Narset, Enlightened Exile won't affect that ability. The creature will still get +1/+1 until end of turn.