Dumbledore

Albus Percival Wulfric Brian Dumbledore is one of the world's most powerful wizards, proven by how the school under his protection was infiltrated by trolls, exposed children to soul-sucking Dementors, actually contained an entire "Secret room of child-killing evil," attacked by murderous thugs twice (including that time Dumbledore was out drinking with an underage boy), penetrated by his mass-murdering mortal enemy twice (including that time Dumbledore hired him) and once forced students to fight a dragon for the amusement of viewers.

Dumbledore's primary strategy for fighting an invincible force of evil was to hide with a bunch of kids and just hope that one turned out to be a custom-made anti-Voldemort bomb.

According to author J.K. Rowling, Dumbeldore "is the epitome of goodness": demonstrated by his experimentation with endangered animal's blood, planning of a Nazi-style takeover of non-magical humans and how he maybe killed his own little sister.  You know, like all good guys.  Perhaps his most reprehensible act was when he discovered that Harry had a mindlink to Voldemort, and intentionally drove Harry to drown Voldemort and the reader in over eight hundred pages of whining teenage angst.  Alas, only Voldemort escaped unharmed.

Dumbledore had access to some of the most powerful magical artifacts in existence.  Here's how he used them.

Phoenix Tears:  Deus Ex Machina (one use only!)
Time-Turner:  Make an annoying swot even more annoying.
Invincible Elder Wand:  "Screw you guys and your desperate struggle against nigh-omnipotent evil, I'm taking this to my grave."
Invisibility Cloak: Enabling the "hero" to spend most of the books hiding under a blanket.