Solutiion for Blood Sport
The Mercenaries at the Arena will be scaled to whatever your current Rank is, but I'd recommend being at least a Level 21 so you can get at least 1 Arena Champion fight done while your there. (The first Arena Champion is a Level 22)
Follow Apache117x's great guide on Are you Not Entertained? Achievement if you need help finding the Arena.
Once you are at the Arena, speak with Skoura..... He will ask you who you'd like to fight, simply choose the very bottom fighter option. (It will have the Mercenary Helmet Symbol next to it).
As far as Skills go, I'd recommend Second Wind and Overpower Attack but it should be managable on Easy or Normal by Level 20.