Using it in the sun would be ideal, as it'd give it a pseudo-stab, and Ray has either 390 something or 438 Atk, depending on nature.
Using it in the sun wouldn't work because of Air Lock.
While that does sounds nice, I think you're forgetting about Rayquaza's ability Air Lock. It will kill the power boost that you would get from the sun unless you have a way to change/disable that ability.
Not a lot of Pokemon in the Uber Metagame are weak to Fire type attacks. You would be hard pressed to see a OHKO happening.