Explanations

On comparative power

Let me be frank. Even if I take Roy's highest showings, he's still solidly below Cinder.

He can barely destroy a small building with his fire, while Cinder was tearing a mountain apart in her fight with Raven. He can barely char a human corpse, while Cinder can turn one to ashes (RIP Pyrrha) with a single touch and no effort. He can hardly even dodge bullets, while Cinder is casually above people who can deflect them without effort.

So if we put them in a straight contest of DBZ power levels, I'm afraid Roy is outmatched. Thank god I'm not a DBZ fan.

Why Roy didn't burn out Cinder's eyes or something like that.

Roy's powers work by igniting the gases in the air in a trail around his opponent. Aura acts as a shield that covers the body

Why was PreGate Roy used

Do you expect me to put a blind man against Cinder? And if I grant him his eyes, it still doesn't change the fact that he's simply too absurdly slow to get a hit on her unless she's extremely stupid. And I can't use PIS. I have standards.

Briggs

As I said, he's COLONEL Mustang. He has access to anything he can get his hands on, including his men. Cinder is also allowed to do so, but do you seriously imagine her bringing help against a weak mortal?

Knowledge

Both were allowed knowledge on the other's abilities, otherwise one could simply gank the other before they know that they have superpowers. Cinder was smart enough to anticipate anything Roy could think of, and Roy knew her personality.

AN: hope that helped! Please direct further questions to PM.

And do suggest it if you would like me to write any more Crossover fights. I have dabbled in quite a few fandoms, and am willing to think of VS battles. Just send a request and I'll see what I can do.