DYNASTY WARRIORS 6

As is the case with most of the DW6 challenges, Sun Shang Xiang or Yue Ying make this a piece of cake. Just keep your distance from officers as much as possible. Also, after you kill Lu Bu, make sure you park yourself right where he spawned because "enemy reinforcements" will appear there right after he dies - lambs to the slaughter.

If you don't want to take the easy way out, characters with special attacks/musou attacks with a large radius can bring you to pretty high records. A personal favourite for such as task is Zhang Liao.