Barrenton Medic + Agatha's Soul Cauldron + Incubation Druid

Barrenton Medic

Agatha's Soul Cauldron

Incubation Druid

How it works

1. If Incubation Druid does not have a +1/+1 counter on it, activate its adapt ability by paying 3, putting three +1/+1 counters on it.
2. Activate Incubation Druid by tapping it, adding .
3. Activate Incubation Druid using Barrenton Medic's second ability by putting a -1/-1 counter on it, untapping Incubation Druid.
4. Repeat.