To unlock the Blood Sport trophy in Assassin's Creed Odyssey, you need to first find the Arena and then opt to fight one of the Mercenaries on the Mercenaries tab of the menu. At any time there should be one Mercenary in that has a red Arena logo above their head; this person is currently over in the Arena.
The Arena is in Pirate Point, which is the most south-eastern point of the entire map, the eastern-most sub-region of Pephka. It's a level 30 area so I'd recommend leaving this trophy for the later game.
Luckily the Arena itself is a fast-travel point so once you have discovered it you can quickly return to it, even if you are not feeling ready to take it on.
It's worth noting that the Mercenary's defeat in the arena does not count towards the "They Just Want Cruelty" quest which requires you to win 5 battles against Arena Champions; although the option to battle a Mercenary is on the same dialogue tree as the Champions, the Mercenary doesn't actually count as one of them.
Generally speaking, as long as you are around the same level as the Mercenary this should be easier than most Mercenary fights as you are not going to be assaulted by random nearby soldiers or worse, other Mercenaries. The only thing to be aware of is the spike traps on the central cylinders and around the walls; if an attack sends you into one of these it will do additional damage. Otherwise just be patient and once you have defeated the Mercenary, the trophy will unlock.
It's also worth noting that you should get a Cultist clue from the fight as well. I also got the extremely nice Jason's Golden Fleece armor from the bout, which is presumably a set reward.