After the Gestalt tutorial with Snow you will enter a long stretch of cut scenes and directly after a quick 30 second fight against a pack of Pantheron as Vanille and Sazh the achievement will pop.
By the way - if you are struggling with the Gestalt tutorial, use Libra to get the intel on how to fill up the Gestalt gauge. For this instance, we need to defend as Sentinel. So switch to Sentinel and guard auto until it fills up.