Un-fun battle chests, ridiculous ai health..solo imossible for me it seems

Had my 14 year old nephew helping me, who is quite good, and some of the battle chests in town center (plants) were just ridiculous and un-fun.  We eventually got them, but the aim, and ridiculous skill of the AI is making it just not fun.  Also, if I can't expect to be able to complete these solo, even if it takes very long, i will not go for them in the future. What happened to the mode where when a player joins you, it says 'your enemies grow stronger'...Damn but they are just as strong with even one player.  While i expect them to increase in difficulty, realizing that you will never get some solo, ruins it for me.  Why are the ai so strong, and we seem so weak.  Also, even with the perk that increases foot soldier fire rate, it's pitiful.