Overkill

Equipped creature has double strike. (It deals both first-strike and regular combat damage.) Equip (: Attach to target creature you control. Equip only as a sorcery.)

If you would gain life, you gain twice that much life instead.

At the beginning of each end step, each opponent loses life equal to the life they lost this turn. (Damage causes loss of life.)

Flying, lifelink This creature gets +4/+4 as long as you have 25 or more life.

Vigilance (Attacking doesn't cause this creature to tap.) Lifelink (Damage dealt by this creature also causes you to gain that much life.) At the beginning of your upkeep, if you have 40 or more life, you win the game.

Whenever you gain life, target opponent loses that much life.