The best tip I can give you is... donate for max contribution, do all your convoys and get in on the ninja war. Group skills should be as high as possible as they are universal stats and are much more worth it than strengthening. Especially once you reach level 60+, I suggest dropping strengthening as a whole until you've maxed out your group skills.

The arena coins are imo not wroth it - a better option would be to just stack them up - or go for someone like Hidan.