In your journal, the "Cross-Class Combos" page (under the Art of War tab) states that there are several ways to get this achievement.
I'm playing a warrior and used the Stagger/Primal magic method, so that's the method I'll describe.
You'll need 4 ability points for steps 1 & 2.
1. Put an ability point into both the warrior's Pommel Strike ability, as well as its upgrade, Pommel Blow. This ability can be found in the Warmonger tree.2. Put an ability point into both Merrill's Chain Lightning spell, as well as its upgrade, Chain Reaction. This spell can be found in the Primal school of magic (I'll explain how to get Merrill and why I used her at the end, which, just in case, is listed as a spoiler).3. This step is optional, but just so I didn't have to think about it in combat, I set one of Merrill's tactics like this:Under tactics, choose an empty (Target):(Condition); go to the 'Enemy' tab; choose 'Status'; choose Staggered. Then go to its corresponding (Action) tab, choose 'Use ability,' then choose 'Chain Lightning.'
Her tactic should read Enemy: Staggered --> Chain Lightning. I also moved this particular tactic to the #2 spot on her tactic list.
4. Enter combat.5. Use your pommel strike ability.6. Merrill should then use her Chain Lightning ability automatically, and your cheevo should pop.Other info:
Thanks to connectmeplease for confirming that you do indeed need to upgrade both Pommel Strike and Chain Lightening before you can get the achievement through this method.
You will recruit Merrill through the Main Plot quest
Lastly, I used Merrill because she starts with two or three ability points for you to distribute when you recruit her, so you can activate and then upgrade Chain Lightening immediately. I'm sure you can also use Bethany, but B lacks the Primal school, which means you need to level her up until you can unlock Crushing Prison in the Arcane school. I found it faster to just recruit Merrill.
Edited for typos and clarity.