Baldur's Gate 3

You can get this achievement anywhere but if you want fast and easy wait for act 3. Go to the Elfsong Tavern and speak with the Chef Roveer in the kitchen. He will ask you to get rid of the rats in the basement. You only need Shadowheart. Take a couple of bottles of wine and enter the basement. Drink a bottle of wine and activate her Spirit Guardian Radiant damage ability. Just stand there while the rat run toward her and die. Theres about 30 rats so the achievement will pop before they are all dead.

When you visit Shar's Gauntlet in Act 2 you will soon run into groups of rats which you could either talk to (with a potion of Animal Talking - to find out that there is something special about them) or wait till you run into a special devil in the depths of this dungeon, who tells you about the curse, that is responsible for their strange behaviour. I won't spoil to much about it, but you can begin to kill these rats (they are not hostile per se) until you get another, last warning. If you kill the "messenger rat" as well, there is the possibility later on to get to the feet of the massive Shar-Statue, that looms over the final area in this dungeon.

When you get there you will be confronted with a massive horde of rats, that will be swarming your position. Give Shadowheart something alcoholic to drink in the inventory (best something that holds for more than 2 turns, but 2 turns were enough for me), activate Spirit Guardion (as mentioned in the other guide) and just wait until they are all killed ny this aura. It's enough if you keep your party close enough together, that the rats need to enter "the Zone" to reach your party members but you can also think about sending Shadowheart in alone. Maybe buff here to keep the concentration because some of the rats are necrotic rats and explode, which can break her concentration.

You also get a nice and handy reward for completing the quest as well.