Pitiless Plunderer triggers, creating a Treasure token.
Resolve the
Garna trigger, returning it from your graveyard to your hand.
Activate the Treasure by tapping and sacrificing it, adding ({R} magic symbol) .
Repeat.
Notes
You can repeat this loop with any number of additional creatures. This will require the desired creatures to either be on the battlefield already, or to be in your hand with an additional ({R} magic symbol) available for each creature. Simply play all other creatures with
Sneak Attack before
Garna, and then make sure to sacrifice them all for ({R} magic symbol) before resolving