This achievement can be a little tricky in terms of creating the correct criteria for the Cross Chain to occur. Essentially, you need to put an enemy in a state of 'vulnerable' and then allow the red arrow to appear above them as well as the 10 sec timer bar which appears at top of the screen. Once this occurs warpstrike to the 'vulnerable' enemy using
and then complete the following quicktime event using
in time with the on-screen prompts.
I found the easiest enemies to achieve this with were the Garulas (Lvl 10 and over) and Magitek Assassins (Lvl 47 I think) around the campsite near the Chocobo Farm. Make sure you equip the weakest weapons so not to kill the enemies too quickly. Don't worry about killing the enemy with the warpstrike that initiates the Cross Chain as they won't take damage from it until the quicktime event if finished.
For the Garulas, simply warpstrike one from a distance without alerting it prior to the attack. If you're lucky you should get the prompt for the Cross Chain, if not simply warp out of the fight area and let the Garula heal before trying again. For the Magitek Assassins, Gladio will usually hit a group of them in one strike putting a few of them into 'vulnerable' state allowing you to complete the Cross Chain on one of them. Just like the Garulas, if it is not successful, just warp away, let them heal and then try again.