What should do a KI84 vs Spit16? I want know tactics, tricks and maneuvers to be a good pilot of KI and not use it how a spit with more WEP...
Thx for help 
The Ki84 actually has "less" WEP (in the short haul), it just has a faster WEP recharge rate than the 16.
As others have said, during an OTD fight with a 16, you're not going to find any advantages other than sustained turn. Luckily, most 16 pilots are quite awful and will play into your game. But don't be satisfied by a mere ~200mph luftberry, get it as slow, as quickly, as you can. The 84 and 16 will have similiar sustained turn without flaps, but once you can pop your flaps out (very slow in the 84, less than 150mph if I can recall), you'll have no difficulty turning inside.
One other item of note, someone mentioned earlier that the Ki84 is a better E retaining plane. I don't have the data to back myself up, but I can almost certainly say that is false. Spit 16's retain E almost as well as P-51's, from what I've seen, not to mention that the sturdiness of their airframe allows much more E to be built up, whereas the Ki84 starts to threaten to lose parts near 400mph indicated. Overall, E retention is at the very least equal, but I would bet that it's in favour of the Spit 16.