This achievement should unlock naturally during main story. For a long combo, you should find a strong enemy alone or with weaker ones that you can get rid of quickly. Now, the trick is having a 30-40 hit combo and use Alphen's bonus attack to stun your opponent and then, give all the hits you need as fast as you can. If you use skills, you should alternate 4 of them to not trigger penalties.
A great enemy is a dragon with a glowing orange weak spot on the chest. Each hit on this spot counts double as you're hitting both the spot and the enemy behind at the same time.