Put a +1/+1 counter on a creature you control.
Shalai and Hallar triggers, dealing 1 damage to an opponent and causing you to gain one life.
Celestial Unicorn triggers, putting a +1/+1 counter on itself.
Repeat from step 2.

Infinite +1/+1 counters on Celestial Unicorn

Infinite damage

Infinite lifegain

Infinite lifegain triggers

Shalai and Hallar

Celestial Unicorn

Activate Celestial Unicorn using Spike Feeder's second ability by removing a +1/+1 counter from it, causing you to gain 2 life.
Celestial Unicorn triggers, putting a +1/+1 counter on it.
Repeat.

Combos with "Celestial Unicorn"