You can't. And i doubt they'll 'fix' it because that's the way they designed it to be with the new 'Matchmaker' update. Sucks, don't it?
I just plain stopped playing since i can't select a non-pro battle arena. If enough folks quit playing, maybe they'd decide to bring non-pro battle selection back.
I also experienced this problem. Battle would begin loading and bar would get to about 1/3mm from end and no further. A failure would appear telling me that i was removed due to inactivity. My internet speed is good, i ensured i had the latest Adobe Flash and even cleared the cache. Nothing worked. I normally run all programs and games (including writing this) in Firefox. I switched browsers to Internet Explorer and tried loading the battle again... it worked!
Apparently, for some reason, the latest Tanki update doesn't like Firefox. Try switching to a different Internet Browser and see if that helps.
Customer support service by UserEcho