Agatha of the Vile Cauldron has power 4 or greater.
You control at least one basic Mountain and at least one basic Forest.
Steps
Activate a basic Mountain and Forest by tapping them, adding ({R} magic symbol) ({G} magic symbol) .
Activate Earthcraft twice by tapping Cinderhaze Wretch and Barrenton Medic, untapping the Mountain and Forest.
Activate Agatha of the Vile Cauldron by paying at most ({R} magic symbol) ({G} magic symbol) , giving all other creatures you control +1/+1, trample and haste until end of turn.
Activate Cinderhaze Wretch 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, Cinderhaze Wretch and Barrenton Medic.