LEGO Harry Potter Collection

Good Dog can be unlocked at the beginning of the fifth level in Year 1, The Forbidden Forest. Since Fang is a playable character in this level, you don't need to have unlocked him yet.

At the start of the level, there is a beehive on the left. Take control of Fang and press to attack the bees which come out of the beehive. Once you've defeated twenty bees, the achievement will unlock.

Destroying the beehive will stop any bees from being released. If you destroy it before unlocking the achievement, don't worry as there are a few more in the level which you can use.

Easy way to get this is on the 5th main story mission The Forbidden Forest

At the start of the level there is a bee hive to the left. It will have infinitely spawning bees. Just kill 20 with Fang (the dog) and the achievement will pop.

If you destroy the bee hive the bees will no longer spawn but there is another hive further into the first area, another in the second area and a final one in the third area of the level.