Trulon: The Shadow Engine (EP)

This achievement is easily doable around Level 24-25. I would suggest doing this in the Step 1 of the Road Map, and kill everything in the game except the final boss (you can beat him in the areas leading to the rooftop, then just leave). You need to make sure you've completed the "A Job to Kill For" quest to get Detonating Bolts, and got the "Below the Belt" item from the enemies in the Undercity. Equip Gladia with Detonating Bolts, as well as your highest +Damage items.

For cards on Gladia, you'll want two Haste cards, Furious Missile, Furious Missile II, and Below the Belt. Equip Ferra with Haste, Blast of Frailty, and Invigorate. Equip Valencia with Soothing Wave. Take all other cards off to ensure you get all these on your first turn. Now, do things in this order:

• Roth: Just use a Defensive card
• Valencia: Use Soothing Wave
• Ferra: Use Haste, then Blast of Frailty, then Invigorate on Gladia
• Gladia: Use Haste, then Haste again, then both Furious Missile cards, then Below the Belt on a stunned enemy

I got nearly 3,000 damage with this method, so you should have no issues. If for some reason you're not strong enough, finish getting to Level 30 and try again.