Gravitic Punch + Kalamax, the Stormsire + Insidious Will

Prerequisites

You have not cast an instant spell this turn.

Steps

1. Cast Gravitic Punch by paying its mana cost, targeting Kalamax.
2. Holding priority, cast Insidious Will by paying its mana cost, targeting Gravitic Punch.
3. Kalamax's first and second abilities trigger.
4. Resolve the first ability, creating a copy of Insidious Will targeting the original Insidious Will.
5. Resolve the second ability, putting a +1/+1 counter on Kalamax.
6. Resolve the Insidious Will copy, creating a copy of Insidious Will, targeting the original Insidious Will.
7. Kalamax's second ability triggers, putting a +1/+1 counter on itself.
8. Repeat from step 6 any number of times.
9. Once you have an arbitrarily large Kalamax, have the final Insidious Will copy target Gravitic Punch. This'll create two copies of Gravitic Punch in addition to the original Gravitic Punch.
10. Resolve all Gravitic Punches, dealing infinite damage to each of up to three opponents.