Dawnsire has twenty or more charge counters on it.
Steps
Declare a creature as an attacker.
Dawnsire triggers, dealing 100 damage to itself.
Dawnsire dies as a state-based action, triggering
Resourceful Defense, putting at least twenty charge counters on
Darksteel Reactor.
Darksteel Reactor triggers, causing you to win the game.
Notes
You can instead use
Resourceful Defense's activated ability to move the counters. This adds ({4} magic symbol) ({W} magic symbol) to the mana cost, but negates the need to attack.