I'm playing Pokémon showdown and am trying to make a good non hackmon team with landorous-therian and explosion but I cant remember if there is an ability that removes the immunity from normal...
The ability you're thinking of is Scrappy, which allows the user to hit ghost-types with normal and fighting moves.
Alternatively, you could also use Aerilate, which changes the user's normal-types moves to flying-type, which lets you hit ghosts and gives you STAB on Landorus.