In Gen V there is an Item called the bulls eye on serrebi it says that if a move doesn't hit the target becuase the type is immune it will hit. So with Mold breaker and bullseye Haxorus could hit any pokemon with earthquake. I have also heard that is removes the holders immunities but Haxorus has no immunities. If it does nullify the opponents immunities then he can destroy Skarmory
Bull's Eye makes the holder lose their immunity. It's one of those items that are only there to be tricked, and even then there are better things.