Phyrexian Altar + Kithkin Spellduster + Hardened Academic

Phyrexian Altar

Kithkin Spellduster

Hardened Academic

How it works

1. Activate Phyrexian Altar by sacrificing Kithkin Spellduster.
2. When Kithkin Spellduster dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. Hardened Academic triggers, putting a +1/+1 counter on Kithkin Spellduster.
4. Repeat.

This ingenious loop turns a modest persist creature into an infinite resource engine by letting Hardened Academic cancel out the drawback of Kithkin Spellduster. Every time you sacrifice Kithkin Spellduster to Phyrexian Altar for mana, its persist ability brings it back with a -1/-1 counter, but Hardened Academic's triggered ability steps in the moment the card leaves the graveyard to place a +1/+1 counter on it instead. Because +1/+1 and -1/-1 counters annihilate each other as a state-based action, the Spellduster is left completely clean of counters and ready to be sacrificed all over again.