The best Spy is one who is never seen. I love playing Pyro against DR spies, as I can always see them coming. Flame them, and even if they are using spycicle, 99% of the time I know where they`re headed because I know where the ammo is located.
I played some matches as Pyro against Stabby Stabby, and learned a lot. He managed to jump-stab me off an incline so tiny I didn`t even think to see it coming. Now when I see a Spy start to jump for a stair-stab, I airblast instead of flaming. So anger, much burn, very ragequit. Wow.
Same for matador stabs. Awptimus was always a fan of those at practices when he was around as a Recruit, and so I would airblast him out of position and make him miss before sending his burning ass back to the pits of hell from which all Spies are spawned.
Some of the most challenging spies to play against run with Ambassador and Invis-watch. As a Pyro, it`s hard to get close enough to them to flame, and flares are slow so they can dodge. I`m more likely to just let them go than to chase them as they back-pedal and headshot me to death.
It doesn`t matter what disguise you use, it`s only good as long as people don`t pay attention to you. Better to not be seen at all than to rely on the disguise to get you through.
If you can`t be unseen and are playing against bad players, try luring them to a place where you can trickstab them. Up stairs/ramps, around corners, into water, etc.
Fastest way to get better is to play against good opponents. I am not the best Pyro, but I`m a good Spy-hunter, so, as I said, if you want to play a bit, let me know.