You will be reintroduced to the Magikoopa enemy in this blockade level, which are pretty simple enemies. They stand still and do nothing but fire a blast of magic at you, then they will teleport to another spot nearby. You will have to deal with three Magikoopas in this level.
Since they can be elusive, a bit, due to their teleporting, you might have a difficult time defeating them. You can defeat a Magikoopa by jumping on them, like most normal enemies, as well as with any power-up. In fact, the Fire Flower is probably the best option against them, since you’re fighting them with ranged attacks. Defeat all three Magikoopas and you will earn the Green Star.