This is one of the hardest achievements to complete. You will need to string together a comb of 40 hits without being attacked from your enemies. You may find this a little easier with the weapon upgrades as you can pull off far stronger combos and raise your level faster.
Once you raise your combo level it will turn gold allowing you to pull off stronger hits. You will also be able to take longer breaks between combo attacks on enemies giving you time to think about each move you pull off.
While fighting with the Titans you can build up a high combo level while riding on their backs and hitting over Jokers men. Just take your time and practice.
See Freeflow Perfection for combo combination.