Problem with the transmuting is he always has a moment to stop, focus and concentrate to achieve the effect. And it's virtually always done against inanimate objects that can't fight back. Plus, he requires physical contact throughout to do it. So, it's not something he can easily do right in the middle of a fight, and his lack of using transmutation against enemies in the middle of fights onscreen shows this.
And when it comes to actual feats of strength, speed, durability and overall offensive arsenal, Tony has him beat in every category.
Originally posted by Surtur
Tony one shots him before he can do any real damage.
That's the big issue here. Barring the nuke feat (which seemed more like a nuclear energy absorbing feat than a durability feat, which makes sense given the nuclear origin of his power, plus he again needed physical contact and had a moment to get ready), Firestorm has been dropped by everything from blunt force hits, to regular explosions, to energy blasts, and even arrows on one occasion. So, Tony can definitely take him out with his arsenal.
In contrast, based on the fact that 0:37 did little more than temporarily BFR Tony, he can probably take at least a few fire blasts before it starts to have an effect. And that's not even factoring in shields and armour repair.: