Well, the way I got my 10 arena chests is I stealthed up with Noggenfogger (Small) before the match actually "started", IE right before he actually lays the chest.
Before anyone knows it (Cause usually people dont just jump in, they wait for someone else to) you've got the chest.
A. You're really small so it takes a sec for people to relize.
B. They are afraid to jump in cause they'll get pwned if there are alot of people there.
C. They dont get there in time to stop you.