MTG Combo: Phyrexian Altar + Woodfall Primus + Zameck Guildmage

A couple things to note: The Primus can be replaced with anything that has Persist, and the Altar can be replaced by any sac outlet.

Firstly you need to activate the Guildmage's ability, making it so that creatures that enter the battlefield under your control enter with an additional +1/+1 counter. Then you sacrifice the Persist creature with your sac outlet. The Persist trigger will go on the stack and resolve, bringing the creature back with a -1/-1 counter. However, since the Guildmage's ability also adds a +1/+1 counter, the two counters cancel out and allow you to loop this infinitely.