Activate Ashnod's Altar by sacrificing Dominion Saboteur.
When 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.