It's not really game breaking (unless both sides get invul/last stand AND wither) but probably unintentional nonetheless. So, when you use a blockbuster the new permanent does not apply immediately, there's a certain window when both fighters have no permanents. If you manage to activate another blockbuster during this window, you can get 2 permanents on both fighters. It's easiest to reproduce with Impending Doom + any other Peacock blockbuster, but I think something like Napalm Shower + Silent Scope should also work. Probably Beast Of Gehenna + something something immediately (though it's not practical combo wise). EDIT: I just somehow managed to land 3 permanents on enemy Beo. He probably fizzled his BB before I Gehenna + Bandwagon him.