I think the idea here was to encourage individuals to use every available spot and make sure to deal as much damage as you can. The other point I think was to say to change your carry to a hitman to try to dish out the most damage. Lastly, if he's going to escape then just pray you brought his life down enough.
I'd like to point out that in every situation the preceding challenge wave is actually stronger than the horseman and if you're going to use every spot to dish damage to the horsemam with a hitman carry to fill as many as possible with beavers so your hitman can get more hits.