I'm not sure that it's even possible to use the arena to test the balance of builds - and that's just from taking a look at the supposedly balanced arena classes.
I entered the Arena with a Lizard Wizard and a Dwarf Rogue. Both are level 8.
Lizard Wizard
STR = 24
FIN = 20
INT = 29 + 2 = 31
CON = 24
MEM = 42 (!)
WIT = 24
========
Total = 165 - 60 base = 105
Dwarf Rogue
STR = 25 + 2 = 27
FIN = 31
INT = 24
CON = 27
MEM = 42 (!)
WIT = 24
========
Total = 175 - 60 base = 115
The Rogue gets 10 more Attribute points, 7 more Combat abilities, and 2 more Talent points than the Wizard.
The Wizard gets 5 more Civil abilites than the Rogue.
...These are the preset characters which are supposed to be balanced. If the presets need to have wildly different point spreads to be balanced, what hope is there for custom characters?
