When Rageform enters the battlefield, it becomes an Aura with enchant creature. Manifest the top card of your libary and attach Rageform to it. (To manifest a card, put it onto the battlefield face down as a 2/2 creature. Turn it face up any time for its mana cost if it's a creature card.)

Enchanted creature has double strike. (It deals both first-strike and regular combat damage.)