Mystifying Maze can't prevent the Mage Slayer ability from resolving. It helps to visualise the stack. As you declare your attack, the Mage Slayer ability resolves and is put on the stack. In response, your opponent activates his Mystifying Maze and puts this ability on the stack. Everyone passes, Mystifying Maze's ability at the top of the stack resolves, the creature is exiled. Now we get to Mage Slayer's ability, still on the stack. The creature may currently be exiled, but the ability still resolves (and uses last known information to determine the creature's power and toughness). There is no window to use the Mystifying Maze's ability before Mage Slayer can trigger; at any point before the trigger the creature is not yet an attacker, and is therefore an invalid target.