Well said. Every hero's potential to grow depends on the adversaries he had to overcame. If the obstacles are too easy to master the story won't be fascinating or exciting.
Star Wars would be boring without Darth Vader, The Headmaster would have an easy time without stubborn antagonists like Cassandra or Donna or girls who really need help like Priti or Nina. And I expect an interesting challenge in taming Liz, too.
Although the final foe, Zoe Bates, is still in the making, the current tasks are just what we need to gain experience and bring our skills to perfection.