Most people will think of giant monsters & such. I would make a boss that was just an average sized knight one that you could compare yourself to. But of course he would have a crazy fighting style that kept you on your toes testing your reflexes & skills. Similar to lord gwyn. Which was just a one on one fight with an almost average sized man which for some reason was awesome. I would make sure the arena was completely barren with nothing to hide behind or to run to so youd have to face him no matter what. Also the arena would be anti magic so you couldnt cheap your way through the battle. I wouldnt give him any OP magic spamming abilities or anything like that as well but i would definitely make him an aggressive boss that would punish you for any slip ups also with the ability to do back stabs & parry. It would almost feel like youre actually fighting a real person through PvP. & those matches are always intense if the player is good enough. I think itd be a great fight because itd truly put your close combat skills to the test & the rage would be great.