This can be done at any point in the game provided there's enough enemies but there's one particular spot that makes this very easy.
During the second part of the main game you will get a quest from a female NPC in The Aerie called The Magical Stone. Complete the quest and you'll be transported to a battle with numerous weak shades. Equip your lowest damage weapon and the 100 hit combo is easy to obtain. If you don't hit 100 and the battle ends, you can try again by speaking to the NPC.