I would have an invisible spawner wherever you want to put it (in Ultima Online for instance, the monster spawners are little stone graphics with properties telling it what to spawn and how many, which is where I got the idea from). For a platformer it's no different. Just have a spawner sprite, and if it's spawned an enemy, have a variable or boolean switch on so it won't spawn any more, and if the enemy spawned is dead, switch the var back off so it can spawn another enemy.