Just play vs legendary duelist at lv 10 with a deck using only monsters with higher def and lv4 or less, then in your turn put it in face down def. and wait your oponent attack your monsters, always a lv10 legendary duelist will attack your monsters and he gets battle damage lp, so repeat the process until your oponent get zero lifepoint in his/her turn. This way just took me no more than 5 min.
its cumulative points, not PER duel. Do read the condition carefully.
its actually insanely stupid that you're REQUIRED to spend so many gems on one specific mini box, just to get power tool dragon so you can do stage 15, absolutely idiotic who ever made that mission should be fired.