Use them as big bow units or keep them deep with nets and fanatics to tie up powerful enemies. The problem with your plan is twofold. It relies on goblins to do serious fighting, and they can't fight their way out of a wet paper bag. The second problem is you count on maneuvering a horde unit to the flank which is near impossible against an opponent with any experience. Keep goblins for what they're good at, dying so that more orcs may live to win the game. But don't underestimate their toys, nets, fanatics, and huge bow units with poisoned shooting are dangerous.