Tidespout Tyrant + Grim Monolith

Prerequisites

You control a nontoken permanent that can be cast using {C}.

Steps

1. Cast Grim Monolith by paying its mana cost.
2. Tidespout Tyrant triggers, returning a nontoken permanent that can be cast using {C} from the battlefield to your hand.
3. Activate Grim Monolith by tapping it, adding {C}{C}{C}.
4. Cast the nontoken permanent returned in step 2 by paying its mana cost.
5. Tidespout Tyrant triggers, returning Grim Monolith from the battlefield to your hand.
6. Repeat.