Dragon Slayer is a Genshin Impact Challenger: Series VI Achievement that you can get by attacking the core of the Aeonblight Drake's wings, bringing it down in a single challenge. To unlock it, you must challenge the Aeonblight Drake in its cave underneath Devantaka Mountain—and make sure to have a bow user on your team.
How to unlock the Dragonslayer Challenger: Series VI Achievement in Genshin Impact
Start by heading to the Aeonblight Drake's cave in the depths of Devantaka Mountain. You must challenge the boss and wait for it to take flight to attempt the Achievement. As such, you absolutely need a bow user on your team. Additionally, if your team is super strong, we highly recommend not attacking the boss, so you don't accidentally kill it before it starts flying.
The boss has a predetermined move set where it will eventually start flying at one point in the battle, regardless of its HP level. You must wait until the Aeonblight Drake starts flying so that it spreads out its wings and reveals the two cores connecting the wings to its body (circled above). Then attack each core with an Aimed Shot so that they fall out of the Aeonblight Drake, forcing the boss to fall to the ground. You'll unlock the Challenger: Series VI Achievement called Dragonslayer when that happens.
Check out our guide on How to get The Smell of Grilled Mushrooms in the Morning Sumeru Achievement in Genshin Impact to complete another Challenger: Series VI Achievement.