In the Forbidden Corridor, here are the correct spells to use on each obstacle: 

Devil's Snare- Incendio Duo/Tria 
Knight/Rook- Incendio Duo/Tria 
Bishop- Flipendo Tria 
Prof. Quirrel- Incendio Duo/Tria 
Voldemort- Flipendo Tria 

Submitted by james.clag at btinternet.com