It's fairly simple to get this during the prologue. The prologue gives a tutorial to the game's combat system. At the beginning, follow the tutorial, killing the enemies in the vicinity. After a couple of cutscenes, the game takes you outside where it teaches you additional skills.
You level up very quickly acquiring additional skills as you level. You've got unlimited magic and plenty of enemies confronting you so you're ideally situated to go after the Combo Master achievement for pulling off a 100 hit combo. Just use the skill you are most comfortable with and keep an eye on the counter.
If you are still getting acclimated to the game and do not get the combo counter up to 100, you lose very little time by quitting the game, starting a new one, and trying again. In fact, obtaining this achievement during the prologue is a good indicator that you are prepared to play the main game.