When Rageform enters the battlefield, it becomes an Aura with enchant creature. Manifest the top card of your library 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.)