Starfire tears him down from the sky if she gets a hand on him. He can absorb her blasts, but she can absorb his radiation while shrugging off heat and flame.
She didn't die when she got hit with the Astro Force, so maybe. Extreme heat and cold don't really bother her either. Even if she couldn't, I don't think Sunfire would fare better against her nova blast better than Braniac did so it's a ranged stalemate.
If it boils down to range attacks I give it to Sunfire. If Sunfire is stupid enough to ignore the fact his powers are most effective at range against Starfire and let's her get in close then it's his loss.
I see it working out like this. Theia has a hard time even hitting her, and then takes one in the face. That'd be the part where Sunfire goes down. If not, you'll notice Starfire tanking Theia's blasts where Donna is not thought likely to survive. And Donna survives nukes n shit.