Flaming Fist

Commander creatures you own have "Whenever this creature attacks, it gains double strike until end of turn."

Twinblade Blessing

Flash (You may cast this spell any time you could cast an instant.)
Enchant creature
Enchanted creature has double strike. (It deals both first-strike and regular combat damage.)

Duelist's Heritage

Whenever one or more creatures attack, you may have target attacking creature gain double strike until end of turn.

Arashin Foremost

Double strike
Whenever Arashin Foremost enters or attacks, another target Warrior creature you control gains double strike until end of turn.

Destroy Evil

Choose one —
• Destroy target creature with toughness 4 or greater.
• Destroy target enchantment.

Battle Mastery

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

Avenging Huntbonder

Double strike
Whenever Avenging Huntbonder attacks, put a double strike counter on another target attacking creature.

Avacyn, Guardian Angel

Flying, vigilance
{1}{W}: Prevent all damage that would be dealt to another target creature this turn by sources of the color of your choice.
{5}{W}{W}: Prevent all damage that would be dealt to target player or planeswalker this turn by sources of the color of your choice.