Agatha of the Vile Cauldron has power 7 or greater
({2} magic symbol) ({G} magic symbol) ({G} magic symbol) available
Steps
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