The developers of Baldur’s Gate 3 designed most goals so that they may be completed in multiple ways. Thus, giving the players more freedom in how they approach situations. This guide represents one method that is confirmed to unlock this achievement.
This achievement is a bit ambiguous. You can’t simply use an enemy to attack another enemy with an ability like “Throw”. You need to use the ability “Improvised Melee Weapon” to pick up an enemy and use it against another enemy. This should be done with a strength-based character with a good chance of succeeding with an athletics check. Lae’zel is a good choice unless you rolled a strength-based character with athletics proficiency.
This can be earned anywhere in the game. A good place to get this early on is in the Goblin Camp. There are 2 guards standing next to each other in the doorway of the camp.
Save the game before interacting with them. Then, select Lae’zel (or another strength character), choose Improvised Melee Weapon, pick one of the goblin guards to use as the weapon, and end with selecting the other goblin as the target. If the athletics check fails, reload the last save and try again. Once the check succeeds, the Two Birds with One Gnome achievement will unlock.