Dominion Saboteur dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it. As it enters, choose to have it copy a creature you control that has a +1/+1 counter on it, so
Dominion Saboteur will also enter with a +1/+1 counter on it.
Resolve the
Hammerhead ability.
Repeat.
Results
Infinite +1/+1 counters on a creature.
Infinite creature ETB.
Infinite creature LTB.
Infinite creature sacrifice triggers.
Infinite death triggers.
Metadata
In 0 decks according to EDHREC.
Bracket Tag
This combo has been estimated to be Exhibition (Bracket 1+).