Activate Castle Garenbrig's second ability by paying ({2} magic symbol) ({G} magic symbol) ({G} magic symbol) and tapping it, adding six ({G} magic symbol) that can only be spent to cast creature spells or to activate abilities of creatures.
Activate Silvanus's Invoker by paying ({1} magic symbol) , untapping Castle Garenbrig 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 if you control lands that can produce ({R} magic symbol) and ({G} magic symbol) , 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.