Agatha of the Vile Cauldron has power 1 or greater.
({1} magic symbol) ({U} magic symbol) at most available.
Steps
Activate Katsumasa by paying at most ({1} magic symbol) ({U} magic symbol) , causing Basalt Monolith to become a 1/1 artifact creature until end of turn.
Activate Basalt Monolith's first ability by tapping it, adding ({C} magic symbol) ({C} magic symbol) ({C} magic symbol) .
activate Basalt Monolith's second ability by paying at most ({2} magic symbol) , untapping it.