The hangup is what you mentioned where people can farm spawned enemies in boss fights. This could be remedied by having a 30 minute timecap or something but that seems rather arbitrary. Likely the highest score is gotten from using no special weapons which will add a challenge as well and along with a 30-min limit might be reasonable. Either way people will farm Kid for points which makes it tough to consider this as a category even though it would be really fun. An honor system may work.