Chapter 12 – Walkthrough
I made sure the MC, Diana, and Abygale all had two healers in their group. Two healers helps a lot as far as keeping up with the damage dealt. Then I prioritized attacking the cannon units and letting the non-cannon units attack me instead.
Cannons can’t attack back if they’re defending, so you have an opportunity to eliminate them without them damaging you. Archers help a lot on that, as do Stealth units, since both can attack the Cannons in the back row without having to take out the Sentinels first. Abygale with two dragon units can eliminate a cannon squad in one go, so put her on one side and MC and Diana on the other.
The other thing to keep in mind is that the AI Cannons have 3 range, but yours will only have 2 unless you have the range upgrade for them.
One thing that really helps for getting the achievement is to do it on the lowest difficulty.