Blessed hammer is magic damage, there is no resistance for that.I know of at least 1 item with magic ob but it's a useless item stat without the resists to go along with it.Try getting your max health up, have any life scs?If not that's where I'd start.
EDIT: damage is reduced by a percentage in pvp (before items with the damage reduced by% mod), I don't remember what the percentage is but it's a pretty steep, that's why your 4k hit didn't one shot your friend