Grist in exile (exiled by Agatha's Soul Cauldron).
Agatha's Soul Cauldron and Cathars' Crusade on the battlefield.
You control a creature with a +1/+1 counter on it.
Steps
Activate a creature you control using Grist's first loyalty ability by putting a loyalty counter on it, causing you to create a 1/1 Insect creature token and mill a card, and then put a loyalty counter on the creature and repeat the process if the card was an Insect.
When the Insect enters the battlefield, Cathars' Crusade triggers, putting a +1/+1 counter on each creature you control and causing the Insect token to gain Grist's abilities.
Repeat from step 1 using the Insect token instead.