I played an evil run on my second play through. In act 3 I randomly walked into a fireworks shop and it didn't take long for me to realize what must be done. I managed to clear the building and scavenge every firework, loose or boxed, into my stash. After finishing up all my loose ends in act 3, I hauled as much as I could with the mighty cloth on my barbarian to the final fight.
After clearing the way to the portal, I gathered everyone I could inside the portal, laid down every explosive on hand and sent a lone little rocket to set it all off.
I want to say it looked amazing but my computer couldn't handle all the fireworks going off all at once and lagged hard. However, I did one-shot the Netherbrain on turn one and accomplished exactly what I wanted.
Spoiler
That's a 2-football field diameter, for those who think in American