So does that mean the second attack can be with fire?
Or do they need to make one regular attack, then one attack with something other than fire (punch it with their fist, for example), and then they can use fire?
What sort of performance is required to satisfy the other people at the table that you haven't metagamed?
Maybe everybody should roll a d8, and whatever number they get, on that round they can use fire?
You're describing a grotesque situation but I can assure you that at my table it works perfectly well and is fun and smooth. I have no idea why this seems to bother you so much.