There's a total of 5 Metalback spiders in the "High Caves" level.
3 of them are located in the first accessible cave -- these 3 can be defeated by using the supercharge move (sprint over arrows on the ground).
The next 2 are located farther in the level; these 2 are killed by using a boosted/superflame ability. The superflame ability is acquired by talking to the fairy (simply walk up to her and a cutscene will start)
Once you have the superflame, go back into the cave and defeat the remaining two spiders.