For your leper? Natural swing is fantastic for him if you can get it. A lepers accuracy is his weakness, so giving him anything to boost it will make him into a walking wood chopper. Anything else would just be anything that makes him tougher. I cannot remember the name of the quirk, but something that helps his bleed resist is highly recommended. He has the worst bleed resistance out of all the characters.

Do you need to fight a shambler? No. You can play the entire game from start to finish without ever fighting him and not suffering for it. It can be worth it for the rewards, but you need to weigh the cost benefit and your willingness to to engage with it.