Here is a simple guide for finding the first 5 challengers for Defenders of Neopia, however, fighting them is up to you!!!
Mission 1
Enemy: The Pant Devil
Location: Anywhere (wait until he robs you, then fight him)
Tips: None (this is a basic fight for experienced pets)
Mission 2
Enemy: The Ghost Lupe
Location: 131 Soup Alley (Click, in this order: “Pet Central”, “NeoHome”, “Neighbourhood”, “Neopia Central”, “Soup Alley”, then enter 131 as the address and he should appear.
Tips: None
Mission 3
Enemy: Cave Chia
Location: Cave Paintings, page two (You may have to refresh it several times)
Tips: He doesn’t use anything to special, no Downsize Power Pluses or the such
Mission 4
Enemy: Meuka
Location: Quick Ref Page (however your pet must be sick at the time, I suggest trying the Wheel of Excitement, the Skull will give you the correct illness, but be prepared to make several visits)
Tips: This was a hard fight for me, as he uses Radioactive Muffins and Downsize Power Pluses. Be prepared.
Mission 5
Enemy: Brain Tree
Location: Haunted Woods (You must complete his quest to fight him, however you must complete two Esophagor quests for the answers for the Brain Tree quest)
Tips: As he has a large supply of HP, Drain life is a valuable ability. – lemmose