Charnelhoard Wurm as an attacker, triggering its firebending ability, adding ({R} magic symbol) ({R} magic symbol) ({R} magic symbol) ({R} magic symbol) ({R} magic symbol) .
Holding priority, cast
Savage Beating by paying ({3} magic symbol) ({R} magic symbol) ({R} magic symbol) , causing you to untap all creatures you control and get an additional combat phase after this one.
Deal combat damage using
Charnelhoard Wurm, triggering its ability, returning
Savage Beating from your graveyard to your hand.
Repeat from step 2.
Notes
You can attack with additional creatures for additional ({R} magic symbol) and choose more modes using Great Train Heist.