i'm currently boosting 2x Druids (40 atm) with a 70 lock. The EXP for kills is reduced, but the main bulk of EXP comes from quest turn ins (not affected by a 70 boost if your account is not RAF linked on the booster).

e.g.. pickup quest with druids, run around killing mobs with lock (Seed of Corruption & Drain Life FTW) complete quest and turn in. rinse and repeat, you can also run them through instances, but i have found this to be mind numbingly boring (unless its the ZF stairs :thumbsup: ), so am sticking to questing