It is possible to get a 100% kill rate in this mission. Just before you approach the last group of enemies in the lower right corner, do not approach them with your main character. Instead, position your main character as far away from them as possible then send your second character over just close enough to have them chase him or her. Lure them to where your first character is waiting and slaughter them. After that, finish the stage with the fight with the main Boss. Provided that you killed all the other enemies, you will get a 100% kill rate at the reward screen.