That 20mm in the back can be killer if you are creeping up from behind.
Not if you stay below. The 20mm is in the top turret and can't depress below level.
At lower altitudes I do indeed the standard 1-2 oclock high attack. At 30k however, this could indeed take too much time to setup, and there's a much bigger danger of getting into the top turrets field of fire at that altitude as your maneuverability is significantly decrease. And with the Ki the #1 rule is... avoid the top turret!
So if I don't have the time to setup up a high frontal attack (assuming not flying a Komet), I stay below and several hundred feet to one side, speeding up then doing a low 2-4 oclock shallow zoom attack. If done correctly, only the side mounted single 12.7mm can shoot at you. And unlike most US buffs, the Ki gunner has to be somewhat cautious at spending his ammo, he hasn't exactly much of it.