Have duels against Legendary Duelists Lvl 10. They are the weakest opponents in Duel World and require you to spend less Gate Keys.
What does "Use Skill 1 time" mean?
It is very simple, you just need to activate skill one time. If you do not know what skill is and how to set a skill to your deck, see the link below to GameA skill database. I would recommend you just set Yami Yugi's Power of Dark skill or Seto Kaiba's Peak Performance skill since those skills can be obtained when they reach level 4 and activate automatically at the start of a duel.
Just duel a legendary duelist that's at level 10 and build a deck of mostly trap cards and monsters with low level attacks and a few tribute summon monsters. When I had to do it, I set monsters and spammed stuff like mirror wall and staunch defender.