If I really wanted to get rid of recruits fast, I would start an adventure like SE, send every general I have loaded with recruits (or up to however many I wanted to get rid of), attack boss camps, then cancel the adventure when all generals were recovering. This would take 30 minutes travelling time (for the slow generals) plus maybe another 15 minutes attacking etc. and I would get a fair amount of XP (or star coins).
Far from an ideal solution but it's the best quick-fix I can come up with.