Gravitic Punch + Kalamax, the Stormsire + Return the Favor Combo

Kalamax on the battlefield (Kalamax is tapped on the battlefield).

Gravitic Punch and Return the Favor in hand.

You have not cast an instant spell this turn.

available

Cast Gravitic Punch by paying its mana cost, targeting Kalamax.

Holding priority, cast Return the Favor by paying plus an additional to select its first mode, targeting Gravitic Punch.

Kalamax's first and second abilities trigger.

Resolve the first ability, creating a copy of Return the Favor targeting the original Return the Favor.

Resolve the second ability, putting a +1/+1 counter on Kalamax.

Resolve the Return the Favor copy, creating a copy of Return the Favor, targeting the original Return the Favor.

Kalamax's second ability triggers, putting a +1/+1 counter on itself.

Repeat from step 6 any number of times.

Once you have an arbitrarily large Kalamax, have the final Return the Favor copy target Gravitic Punch. This'll create two copies of Gravitic Punch in addition to the original Gravitic Punch.

Resolve all Gravitic Punches, dealing infinite damage to each of up to three opponents.

Infinite magecraft triggers

Infinite +1/+1 counters on a creature

Infinite damage