In the Pacific, the Betty Bombers are crazy easy to kill, especially compared to the Allied equivalents, as you can see below, I shot down 15 of them in one match, and while I was damaged, it was not critical by any means. The Gun I used mostly to do this was the 7.62 (Should be labeled 303 by the way.), as the 20 mm runs out fast in the boomerang.
The Allied 7.62 has sown its self of late to be way over modeled in terms of effectiveness, not just in the pacific, but also in the Tunisian campaign, since it was apparently silently Buffed.
Another thing the Bettys had a freakign 20 mm Type 99 MK I tail gun, yet I can get very close with the Boomerang and not worry to much about being hurt, A ki 61 on the other hand is lucky to get away alive after just a couple bombers are taken out.
Some of this is of course Highly subjective, I just find it far harder to shoot down the Allied Bombers in the Pacific than I do the Japanese bombers.