For Stage 1-10, I recommend you to use LIGHT monsters, high level monsters, and trap cards to get Mako Tsunami, Ishizu Ishtar, Odion to appear at the Gate earlier. You can easily win victories using such cards since Standard Duelists uses very weak decks at Stage 1-10.
Which card should I trade to complete one of Stage 7 missions?
Just trade cheap card that you can trade spending only Gold and save trader items for getting good cards near future. I strongly recommend you to trade Mythical Beast Cerberus, but you will not have enough trader items at this points.
It means you have to win with 500+ LP to win. If you have Paradox brothers unlocked, you can use their "Elements Unite" skill, which starts you at 500 LP, Gate Guardian in your hand, and Sanga, Kazejin, and Suijin on your field. Then duel at the gate against a lvl 10 duelist. I highly recommend not dueling against Joey or Odion.
I am havin the same issue and I have put in an inquiry with the people who own the game and they keep gettin back to me with the same message not telling me if it's going to be fixed or anything, they just keep saying that there is nothing wrong with thier system and that they ask I understand and keep playing and they also say that they can't disclose any inquires that involve walk through inform