Ironic Kill: In Episode 9 or its Challenges, Punish Giggles with the Discos Toy chest.

Firstly, Giggles is this little cutey-pie:

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You first need to play through the episode (stick to the story episode and not the challenge; its simpler) using Bear Beard. You can use whichever bear you want, but Bear Beard will not be set upon by Pirates, so it can make going for singular achievements like this a bit easier.

Play through the episode until you reach the disco in the third area. At this point, head for the docks. Feel free to kill any and all bears in this area (with the exception of Giggles). You'll notice a blue cabin right next to the pier. Outside the door is a small blue toy chest. You need to sabotage this, and wait for Giggles to come and repair it. Once you have sabotaged it, hide in the long grass nearby. Now it's just a case of patiently waiting for Giggles. The only thing that can ruin your achievement run, is another bear repairing the chest. To avoid this, if you haven't killed them all is to make sure you stay in the grass nearby and do not leave the area. If another bear starts heading towards your chest, panic him so he will run away. As soon as Giggles starts repairing the chest, run up behind him and press RT to punish him.

Here is a video showing you what to do -