Monster Madness by Dean Lorey

It's the monster hunters' last chance to stop an all-out monster war

We're under attack The last of the mega-monsters known as the Named are fighting their way into our world, and only a mysterious being known as the Guardian can help--but the Guardian is dying. To save him, Charlie Benjamin and his friends from the Nightmare Academy must journey into the heart of a Hydra cave, through the spooky Bermuda Triangle graveyard, and across the treacherous, monster-filled San Diego Zoo.