If you are having trouble passing arena mission, try saying that in battle chat and ask for a win. Most people will give you a win if you ask. I often just retreat if I see someone with a kankuro or zabuza in their team, since those are usually only ever used in teams for mission. When the win 15 arena battles event is running, don't try to get it all done on the first day. I usually try to get 3 wins a day when that event is happening, and after every time I win, I will usually retreat and give the next person I match with a win right after. Sometimes, when that event is happening, I will just make a cat team and constantly retreat every fight until I can't get a match in 10 seconds or so.

Arena is a very poor place to test out new teams (so many retreat or just run auto), much better to do "even fight" with a friend on your server. But, if you are just testing a team in arena, when the fight is basically decided, then retreat.