Kalamax, the Stormsire | Expansion // Explosion

Cards

({UR} magic symbol) ({UR} magic symbol) plus the cost of the additional instant available.

Cast any instant by paying its mana cost.
Holding priority, cast
Expansion by paying its mana cost.
Kalamax's first and second abilities trigger.
Resolve the first ability, creating a copy of
Expansion and choosing to copy the original
Expansion.
Resolve the second ability, putting a +1/+1 counter on
Kalamax.
Resolve the
Expansion copy, creating a copy of
Expansion.
Kalamax's second ability triggers, putting a +1/+1 counter on itself.
Repeat from step 6 any number of times.

This combo has been estimated to be Oddball (Bracket 2-3+).