Inside Wimsyshire there is a possibility of a specific geometric formation:
If you drag a boss pack of Ponies to this spot and stand as show in either spot below then you can shoot them (off screen avoids things like mortar) without them following you or taking any damage.
Out of the 3 times I've currently done this it has had this formation in twice. Not a foregone conclusion as to if this formation is present in most games however its a start.
From above (harder to keep track of health)
From below
Using this has allowed me to kill packs that would usually be impossible or very hard. Wont help with other packs but at least you can get dem Ponies! (I find them to be the harder of the 3 types of packs due to speed) From what I gather the AI considers the path around the formation a valid one however they cant fit due to thier hitbox being too large and the AI does not override this and make them walk round.
Video of this happening was recorded on my twitch.tv channel here actionpigeon - Wimsyshire!
This is very specific so I'm sorry this wont be useful to everyone, I dont know of any melee farming Wimsyshire anyway
Cheers and GL