Grist in exile (exiled by Agatha's Soul Cauldron).
Agatha's Soul Cauldron and Metallic Mimic on the battlefield.
Metallic Mimic has at least one +1/+1 counter on it.
"Insect" chosen with Metallic Mimic.
Steps
Activate Metallic Mimic 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 Metallic Mimic and repeat the process if the card was an Insect.
The Insect enters the battlefield with a +1/+1 counter on it due to Metallic Mimic, causing it to have Grist's abilities.
Repeat from step 1 using the Insect token instead.