so far arena needs a bit of tweaking, like remove the scaling of bosses that spawn and give them a special stat like the spymaster has (can only take damage when emerged). this makes the arena a bit easier but also has not that great of an impact on the gameplay of the arena.
And also a few of the mobs (mostly ranged) need their range of attack a bit lowered because so far ice elementals can almost snipe you across the map, mercenaries crossbowmen need their attack speed lowered a bit, and marsh beetle a bit less tanky but overall the most mobs are balanced.