Master Biomancer has at least one +1/+1 counter on it.
Steps
Activate
Master Biomancer 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
Master Biomancer and repeat the process if the card was an Insect.
The Insect enters the battlefield with at least three +1/+1 counters on it due to
Master Biomancer, causing it to have
Grist's abilities.
Repeat from step 1 using the Insect token instead.