Step 1: Cast Outpost Siege, choose "Dragons."
Step 2: Have Midnight Guard in play.
Step 3: Target Midnight Guard with Retraction Helix.
Step 4: Play Ornithopter.
Step 5: Tap Midnight Guard return Ornithopter to your hand. Outpost Siege deals 1 damage to target creature or player.
Step 6: Play Ornithopter again, making Midnight Guard untap. Use Midnight Guard to return Ornithopter again. Outpost Siege deals 1 damage to target creature or player.
Rinse and repeat. Additionally, you can use these two cards:
Purphoros, God of the Forge would work greatly in tandem with Outpost Siege. While the latter deals 1 damage to target creature or player every time Ornithopter is returned to your hand thanks to Retraction Helix being attached to Midnight Guard, the former would deal two damage to each opponent (hello, multiplayer!) each time Ornithopter enters the battlefield. Moreover, if you can't win through lethal damage, use Altar of the Brood to empty his or her library by bouncing Ornithopter again and again.