Try to keep the fight moving up in the Ki84. Once you've gotten your enemy to follow you up and blow his E you should be slow and able to pop out the wonder flaps. Now all thats left is to simply whip your nose around while the enemy wallows and nail him.
Be easy on the trigger, those 20mms can spit out a lot of ammo fast. I find that I get most of my kills between 200 and 400 yards. The pea shooters aren't completely useless either, I've gotten 2 or 3 kills with them after going bingo on the cannon rounds.
Lately I've been taking 100% fuel most of the time. But if you want the best performance at the start of a fight perhaps consider taking 50% and dts.
The two bombs are nothing to scoff at either, plenty to kill a panzer or m4 with a well placed drop.
Last tip, at the higher speeds up until you hear the plane groaning the flight performance is very good. Feel free to lure high cons into a flat or rolling scissors for a nice overshoot snapshot. The 20mms will leave a hurting with only a few hits. I'd be happy to wing up with you if you see me online, I consider the Ki84 my best plane. (wish i could say this about the p38 but I'm just a gangtard magnet in that apparently)