Into the Shadowlands by Cynthia Reeg
Book 2: Monster or Die!
Troll Malcolm McNastee and misfit Frankenstein Frightface Gordon—middle school monsters at Fiendful Fiends Academy and long-time enemies—venture into the infamous Shadowlands to prove their monster worthiness. Too quickly they discover that the mysterious world between Uggarland and the humans holds horrors worse than any nightmare, and if they make it out alive, they’ll undoubtedly not emerge unscathed!