If your opponent hasn't triggered this by the turn after you set it, don't be afraid to do it yourself if you have a lot of monsters on your side of the field. Suicide attacking with one of your Gemini monsters will trigger the trap and let the rest of your monsters in for massive damage.
Summon a gemini monster and equip it with "Supervise". Then attack a stronger monster your opponent controls with it. When your gemini is destroyed you can destroy all of your opponents monsters with this card. "Supervise" revives the gemini you just used as a pawn so you can attack once again with it.