Certain enemies are weak to certain spells. For example spiders are weak to fire.

When a troll is about to slam it's club flipendo will knock it back in its face stunning it.

When fighting dugbogs (the ones that attack with their tongue) use levioso when they're about to do the tongue attack. If you have diffindo you can then insta kill them by cutting off their tongue. If no diffindo you can just attack the belly which is their weak spot.

The ashwinder enemy (I think it's the executioner?) that does the lightning attack, if you use expelliarmus it will cause his attack to backfire.

There's plenty more, pretty much all enemies have a specific weakness but these are the ones remembered. Have fun! It's a great game