Ki84s are master climbers. It is dragging them straight down that is their weakness. Either they shallow out so as not to over-speed, allowing you to come back up in a better place from underneath them, or they commit to a nose-down chase and still have to slow down, allowing you to force overshoots, or force overspeeds.
The Ki is one of the most lethal planes in this game. Most folks don't take the time to learn it well, but it can just about do EVERYTHING better than most planes. Just not follow in dives.