At least one creature you control does not have summoning sickness.
NOTE: This combo can only persist as long as a creature card is milled with each iteration; as such, the more creatures you can tap, the lower your chances of failure.
Steps
Activate all creatures you control by tapping them.
Resolve the first creature's ability, causing target opponent to mill cards equal to the creature's toughness.
For each creature card milled this way, Undead Alchemist triggers, exiling that card and causing you to create a 2/2 Zombie creature token.
If a Zombie is created, it enters the battlefield, triggering Intruder Alarm, untapping all creatures.