I think I've found something that works :D
MY HYBRID TRISTANA SETUP
So I decided on using my AP set of runes. Mostly just AP there, with mana regen and possibly armor. Then, I got masteries that focused on AP but also gave good boosts to AD.
In game, my normal item order:
Doran's Ring
Gives a solid base for my rocket jump and explosive shot.
Berserker's Greaves
Don't want to leave basic attacks out, so getting that extra damage quickly is nice.
Hextech revolver into Hextech gunblade
More nice AP, should definitely have ult by now, so that's a nice boost. Its slow effect can be kinda handy as well. Once you turn it into gunblade, there's also that extra AD and such. That's quite nice.
Lich bane
Mana regen, slight movement speed boost, AP, magic resist, and its passive effect. What more could I want other than health?
Rylai's
Well, at this point I'm going to need some more health, so I figure that at this point my auto attacks are doing decent damage so I can work with AP. Also, the more slows, the better.
Phantom Dancer or Rabadan's
I can't decide, as I haven't had the chance to finish building yet. I've only played a couple games with this setup.
If there are any really obvious flaws, please do point them out.