A monster arena lies off to the side of the Calm Lands, wherein you can battle monsters that you've caught and gain rare items. Sometimes the arena owner "breeds" a new type of monster: a bigger, stronger and much harder monster than normal. In short, the battle arena is a grea tdeal of fun, but has a tendency to be tough too.
New tonberry trick, submitted by firstname.lastname@example.org:
I found an area quite easy to encounter them; got 9 of them within 1 and
1/2 hrs. The area's location is described as followed: turned right at the first
turn, continue going straight till the 2nd intersection and turn right.
walk along the path till you enter a dead end where a treasure chest
can be found on the left hand side, there are spirits flying at both left
and right of the main screen. head to the end of the area you should
see yourself entering a rectangular area which is not shown on the mini
map. this is the area i found quite easy to encounter tonberries. i then
use analog stick and make myself circleing in that area till i
encounter monsters. i also fleed all other monsters i've encoutered.
i hope this info can be useful for everyone who has problem finding
those bloody tonberries :)