Crackdown Construct and Gigantoplasm on the battlefield (Gigantoplasm as a copy of Crackdown Construct).
Steps
Activate Gigantoplasm by paying ({0} magic symbol) .
Crackdown Construct and Gigantoplasm trigger, giving themselves +1/+1 until end of turn.
Repeat.
Notes
This combo will also work if Gigantoplasm is already on the battlefield as a copy of a different creature. To prevent Gigantoplasm dying due to having 0 toughness, activate its ability many times while holding priority.
Results
Infinitely large Crackdown Construct and Gigantoplasm until end of turn.