Agatha of the Vile Cauldron has power 4 or greater.
({R} magic symbol) ({G} magic symbol) available.
Steps
Activate Agatha of the Vile Cauldron by paying ({R} magic symbol) ({G} magic symbol) , giving all other creatures you control +1/+1, trample and haste until end of turn.
Activate Devoted Druid and Barrenton Medic by tapping them, adding ({R} magic symbol) ({G} magic symbol) .
Activate Devoted Druid and Barrenton Medic's second abilities by putting a -1/-1 counter on them, untapping them.
Repeat.
Results
Infinitely large creatures until end of turn except Agatha of the Vile Cauldron, Devoted Druid and Barrenton Medic.