In a multiplayer game, if creatures you control deal combat damage to multiple opponents at the same time, the last ability will trigger multiple times
Some of my favorites unique to the deck are Chaos Dragon, Territorial Hellkite, Warmonger Hellkite, and Vengeful Ancestor, which force attacks to trigger Firkraag and continue to lay the smackdown themselves
While in Commander that might not be as high of a cost, in constructed formats like Modern and Legacy, it can be near impossible to do while staying in the game
At this point, the Hero deck would begin to peter out of its supply of Heroes, and the Rat deck would begin to stabilize
In this case, you would draw and discard N cards, and put a +1/+1 counter on the creature for every nonland card you discarded this way