Total War: THREE KINGDOMS

Adding a solution as it appears many abilities don’t work.

Create a custom battle with Sun Ren and some infantry against a weak general (e.g. Pang Tong) with no retinue. Once the battle starts, send forward Sun Ren alone to fight the enemy general. Once the enemy general flees (around half health), select the missile ability (blue icon when Sun Ren is selected) and target the enemy general. They will die and the achievement will pop - only takes 2 minutes.

I want to preface this guide by saying that this achievement might be glitchy, have some strange unlock conditions, or have a really long unlock delay. I've tried for a while to get it with the vanguard and champion's AOE ability during duels with no luck. I'm fairly certain based on timing and animation that it was the killing blow as well. When it finally unlocked for me, the only damage ability I had at the time was Taishi Ci's arrow ability, so I think it might have to be done with a targeted ability. Weirdly, though, I unlocked this achievement in a settlement battle with no enemy heroes. However, my strategist did die while I wasn't paying attention, I suspect by running into Taishi Ci's line of fire. So friendly fire might count?

Generally speaking, try to weaken the hero a bit before using the ability (you can hover over their icon to see how much health they currently have). Duels are a good way of controlling the damage to a single source, but a lot of the targeted abilities I've seen require the hero to not be in a duel, so if that is a requirement, duels may not be a great idea. If the requirements are met, it should unlock during the battle or soon after. Taishi Ci was the character that worked for me, but I think Sun Ren also has a similar ability.