Orthion and Great Oak Guardian on the battlefield.
Orthion does not have summoning sickness.
Creatures you control can tap to produce at least ({1} magic symbol) ({R} magic symbol) .
Steps
Activate any number of creatures by tapping them, adding at least ({1} magic symbol) ({R} magic symbol) .
Activate Orthion's first ability by paying ({1} magic symbol) ({R} magic symbol) and tapping it, creating a token copy of Great Oak Guardian with haste.
When the Great Oak Guardian token enters the battlefield, it triggers, untapping all creatures you control and giving them +2/+2 until end of turn.
Repeat.
Results
Infinite ETB.
Infinite creature tokens with haste.
Infinite untap of creatures you control.
Infinite mana creatures you control can produce.
Infinitely large creatures you control until end of turn.
Infinite copies of creatures you control with haste.