There is a bug in the arena challenges part 3/3 where you have to win 10 defense battles in the english version wich is ok, thing is in the italian version, is different.

In the italian version you have to win 10 arena defense battles in a row, now i don't know wich one of those 2 is the correct one. If you play in the italian version you have to win 10 times in a row, in the english version you just have to win 10 doesn't matter if it's in a row or not so i actually did it with the english version because it was easier. I'll submit 2 screenshots.

In italian "di FILA" means in a row. Hope it helps. Cheers!!