You just need a slave that qualifies... so either 40 ranged or 40 melee or both maybe... I think that is what is needed. I can't remember the exact stats off the top of my head.
You can put and remove a slave, and they will gain a lot of attribute increases. So not terrible to put in some lower tier ones. You can just change it later.
But if you are going for an optimal build up then you want combat slaves like Rebecca or the Valkerie. Personally, I hate putting those more interesting slaves into the arena, but everyone has their own style. I want Rebecca and the valkerie with me... not locked away in the arena.
Maleficent, Brawler, and Steel Maiden are the traits that make the arena trivial. With really strong combat slaves you will just have them fight unarmed to get more coins from victory.
Again, not necessary that you min/max everything.
The game is way, way too easy if you min/max everything. I think it plays better without save scumming and wihtout min/maxing.
Once you get the mechanics it is really easy no matter how you play. You literally can't lose... well I guess you can if you wanted to.