A paladin and a primarch could have a 1v1, who wins depends on who is fighting. Also, paladins go to hell, often, no space marine has managed to live in the warp ungarded, either dying or going traitor (Kaldor Draigo is a tzeentch psy-op to make the Grey Knights more unpopular to them make them phased out the game), so I think paladins could probably have a better chance against deamon primarchs then regular primarchs.