Reaching a combo of 100 in Hogwarts Legacy isn’t very tricky at all. The hardest part is staying in a fight that lasts longer than 100 spell casts, so you can keep that combo alive. That’s why you should sit on this one until later on in the game. This is easily grinded out and possible when you face any kind of troll. 

Usually when facing off against a troll you’d spam magic attacks and use Flipendo to make the troll hit itself in the face, and the fight would be over relatively quickly. However, if you’re one-on-one, just chip away with your basic attack and build that combo up. The massive health bar of the troll makes it easier, along with his telegraphed attacks that you can dodge out the way of. Just don’t get hit, rack up that combo and you'll get a trophy.

And if all else fails, find an Inferius (the manor in Manor Cape is kind of perfect for this), and just keep hitting them with the Basic Case. Because they don't take damage unless you've set them on fire, you can rack up a 100 combo really easily. And you can do it from distance too, so there's literally no chance of you dropping the combo... unless you fall asleep...