Together Forever

You will need to find and pet [npc=180076] twice before getting to KT's room in order for him to be present to do this achievement.

Pet [npc=180076] after The Tarragrue fight. He is located to the right side in the room in the first trash area.

After petting him, he will run off to the next spot.

The next spot is after The Nine, but before [npc=175727]/[npc=176523]/[npc=175729]. You can either clear all the trash on the [npc=175727] side, or have a demon hunter jump over the gap on the [npc=175729] side. After petting him, he will run off again.

[npc=180076] will be located next to the door on the right side in KT's room. Click on him to pick him up. [spell=356347] will put a slight slow on the person carrying him.

The person who is [spell=356347] should be in the Phylactery group. They will have an extra action button to "delivery cat", which will the [npc=176929] the have 500% increase in damage.

Finish the fight as normal to get the achievement.

Video Guide: https://youtu.be/LwWoaNN_Qfo

If by any chance after you get the cat and it despawns, just reset the boss and it'll respawn in the corner

11.0.5 fixed it, it's possible to do it in solo, [npc=203463] will tank the boss for you :)