Activate Silvanus's Invoker by paying ({1} magic symbol) , untapping Gruul Turf and causing it to become an 8/8 Elemental with trample and haste in addition to its other types until end of turn.
Repeat.
Once you have infinite mana, you may tap and untap any other land(s) you control, and use the mana to activate Agatha of the Vile Cauldron's ability.
Results
Infinitely large creatures except Agatha of the Vile Cauldron until end of turn.