After west coast I just use any remaining battles before bed. Sometimes it's zero, sometimes I have as any as 4 if I had to do a refresh during the day. I don't attack anyone in particular, usually try to do teams I rarely get to fight to see how I do against them.
Mostly...I attack the riffraff like the Chinese name and aguermo who constantly attack us allllllll day long.
I think arena is real random. I get those stretches too where I just get attacked like 3-5 times in a row and drop 20 spots.