I learned that sometimes its best to fix your convergence distance, perhaps you were spraying him less effectively rather than a concentrated distance shot.
I have my guns converge at 550-650 depending as I like to take distance shots, getting in some damage on the enemy before engaging into ACM.
With 7.62's, I read that Japanese pilots used these to "bore sight" their target and open up with their cannons shortly afterward. Now I know the JU88 doesn't have cannons, but your best bet is to aim at their ailerons, taking out the tips of his wings. An unskilled pilot will make an evasive manuever and somewhat lose control with that loss. Other than that, pepper the winshield and hope to either make him think twice or hit his pilot.
Although what I always say, better to hit than to miss.