AAing the lower levels, then shooting higher level ships in GB.
Always rotate your support.
If you are leveling a crew that has AA guns in it, mainly KM and IJN, then at lvl 80 stop rotating them. Wait till you hit h39 and yammy and then put them on again.
Quite simply, they get more experience per game than your support, and also they will stay on your ship till BB6. They should hit 115 at about the same time the BO hit 120.
Also level 2 BOs.
It gives much better at-level ship experience with good support and gunners ahead to give a better spread.