The only time I had 88s hit anyone was when I built groups of eight 88 AIs and gave them maximum skill levels. Even then I had to put them right along the bomber path. Lit up the bombers just like real life at 20,000ft
But arena settings for flak are weak at best. Turns out the deadliest are the 50cals which you cant see until they strafe you. The little outposts in Tunisia with four twin 50cals are wicked. Thats why the bombers climb just enough to avoid them. You can tweak the settings for all flak hardcoded to the arena to make them slightly more deadly but even at highest skill levels they generally miss unless you are right on top of them.
Then there is the problem in some terrains like in the ETO where they dont fire at all no matter what you do. I have a theory the when you have too many aircraft and AAA guns in one area the server only processes the important data and the AAA is always last in. But its just a theory.