Agatha's Soul Cauldron + Immaculate Magistrate + Cinderhaze Wretch

Agatha's Soul Cauldron

Immaculate Magistrate

Cinderhaze Wretch

How it works

1. Activate Immaculate Magistrate by tapping it, putting a +1/+1 counter on it for each Elf you control.
2. Activate Immaculate Magistrate using Cinderhaze Wretch's second ability by putting a -1/-1 counter on it, untapping it.
3. Repeat.
4. Once you have infinite +1/+1 counters on Immaculate Magistrate, you may instead target any other creature, and you may also untap them infinitely using Cinderhaze Wretch's second ability.

This clever loop turns Immaculate Magistrate into a one-elf army by granting it the self-untapping activated ability printed on Cinderhaze Wretch, courtesy of Agatha's Soul Cauldron. By exiling the Wretch with the Cauldron and placing a +1/+1 counter on the Magistrate, the Cauldron's passive ability endows the Magistrate with the Wretch's ability to untap itself by putting a -1/-1 counter on itself. Because that -1/-1 counter neatly cancels out a +1/+1 counter as a state-based action, you can tap the Magistrate to pile up counters, instantly remove one to untap, and repeat the process to grow your creatures infinitely.