Ki-45

Ki-45 is the first Japan heavy fighter at tier V.
Quite agile, but with armament good only to attack bombers or other heavy fighters - first that are insignificant 2 machine guns - stock 7.7 or developed 13.2mm along with 20mm cannon - one tier lower light fighters happen to be better armed, and finally it gets 20mm plus 37mm cannon.
Which makes it a tier V version of Bf 110 C-6 - a very random plane often missing when passing light and multirole fighters, or leaving them with few HP.
Airacobras or multirole Yaks also armed with high caliber gun feel this problem much less, as they are armed also with machine guns that can help in stripping target' last HP, and are more agile.
Ki-45 is a sluggish platform, so is okish to attack other sluggish platforms - bombers, heavy fighters, maybe GAA.
Rear gunner is as insignificant as stock armament.
Not a very first choice to grind.