Funny enough, one of my favorite Standard decks I've ever played was a Delirium deck
Early on, it acts as a Force Spike, but later it might as well be labeled as a hard counter
That's just scratching the surface, and it's worth mentioning that, even if it turns into another creature for the turn, Scion still doles out commander damage
Since all the combos require my commander to instantly win, a quick n' easy way to do this would be just to swap Niv-Mizzet, Parun out with a noncombo commander and call it a day: Niv-Mizzet, Dracogenius is a straightforward swap if you still want to keep The Firemind in your command zone, or you can switch to another Spellslinger commander like Kaza, Roil Chaser
On EDHREC they listed Rakdos, the Showstopper and Rakdos, Lord of Riots as the 2 most played general for this tribe