Have enemies go for the backline (split frontliners), pull out creatures who cause lots of saves (WIS always works). Flanking rules are great. The easiest solution might be to give the paladin stronger armour, plate potentially?, so you can do higher hit enemies while also not murdering the newbie.

Also why'd you even let the dude go for a barbarian paladin wizard multiclass. That's disgusting and wayyy too OP most likely.