The series one and series two decks are not created equal. Series two is much more powerful and has far too many immunity situations. I feel like there should be two or three sets of arena duels, one for each deck and one where you could play a combination of both.
It would be handy to have a list of all possible missions, you could rank by time, money, experience, or reward. It is time consuming to sort through each page to find the mission you want.
You can kill grey's by waiting for them to shoot that grey pole of their's and then charging them and slicing. It will take multiple slices with the normal sword. Play them close and you should be fine.
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