Activate Nearheath Stalker using Walking Ballista's second ability by removing a +1/+1 counter from it.
Repeat step 1 until Nearheath Stalker has no +1/+1 counters on it.
Resolve all Nearheath Stalker abilities dealing 1 damage to Nearheath Stalker, and any remaining damage to any target(s).
Nearheath Stalker dies as a state-based action due to having 0 toughness.
Nearheath Stalker's undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Repeat.

Infinite death triggers

Infinite ETB

Infinite LTB

Agatha's Soul Cauldron

Walking Ballista

Nearheath Stalker

Activate Nearheath Stalker using Triskelion's ability by removing a +1/+1 counter from it.
Repeat step 1 until Nearheath Stalker has no +1/+1 counters on it.
Resolve all Nearheath Stalker abilities dealing 1 damage to Nearheath Stalker, and any remaining damage to any target(s).
Nearheath Stalker dies as a state-based action due to having 0 toughness.
Nearheath Stalker's undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Repeat.

Combos with "Nearheath Stalker"