The Ki84 is a mediocre plane at speeds over 170mph when it comes to maneuverabilty although roll rate is very good up to nearly 300 or so. At speeds over 170mph the Ki84 is outturned by all spits, the niki too can turn with it. At speeds over 200 many US planes (depending on fuel status too of course) can turn with it thanks to combat flaps.
The Ki84 has a absolutely supreme climb rate. With 50% fuel the Ki84 reaches 5k in just over one minute. 2 seconds slower then a 109 G10 with 50% fuel (both on WEP). It is one of the few "slow" planes that can totally dictate a fight against many enemies if flown right.
It is not all that slow but it is not up amongst the fastest planes in the game thus it can't run away from most fights using its speed. However, it can climb away.
Slowly getting to my point... The Ki84 absolutely dominates slow speed flying. Bellow 170mph you can pop both notches of flaps. Use this, keep the speed bellow 170 and use your flaps. Keep a cool flying style, specially when engaged with spits as that is most likely the most dangerous plane to the Ki84.
Use its low speed and great climb rate to win.
Look at this film to see what I mean.
Ki84 vs La7, Niki and 109 E4. And here is another one with a slightly different outcome.
Ki 84 vs gang, They've both been posted in the help forum before.