For those struggling with this trophy, like I was, I came across an easier way to get it.
Cross chains take forever to happen, even if you manage to get enemies into vulnerable state. Well there is a trick to getting enemies into instant vulnerable state. Well imperials anyway.
The Noiseblaster
Requirements:
Once you have this, go into the open, like a field somewhere. Every few minutes Imperials/Magitek Assassins will spawn (at least after story they did for me). Once they land, hit them with the Noise blaster, all should fall down and hit vulnerable state, at least one should make the cross chain promote appear. Then just follow the on screen commands.
Do the 10 commands to do once full cross chain attack. Then simply repeat this 10 more times.