Activate Necrotic Ooze using Barrenton Medic's second ability by putting a -1/-1 counter on it, untapping it.
Activate Necrotic Ooze using Fain's second ability by tapping it and removing a -1/-1 counter from it, creating a Treasure token.
Repeat.
Once you have infinite Treasure tokens, you may instead use Fain's third ability to create infinite Inklings, then use Fain's fourth ability to untap Necrotic Ooze.
You then can also use Fain's first ability to get infinite +1/+1 counters.