Naughty Bear

Heres what i did to get this achievement

Unlock Robo Prime by getting gold on 6-1 - 6-2 - 6-3

Then do level 6 with Robo Prime Costume, when you get to the last area, Kill the original Robo Prime.

Injure Fluffy and wait for awhile, eventually for me all the Robo Bears started attacking the normal bears.

I followed fluffy and tryed to direct him in the way of some robo bears, they tryed attacking me but they also tryed attack him, and lets just say hes all fluffed out haha.

Good luck getting this :)

In order to get the RoBo-B34RS to kill Fluffy, you must first unlock one of the robot costumes. Some have suggested you can use the Tech-Naughty costume you get for completing Episode 6 with a gold trophy. However, that costume did not work for me. The next easiest costume to attain is the R0B0-N4UGH7Y costume for getting bronze or better on 6-1 - 6-3, and this one did work.

Dress in this costume and play Episode 6. Once you get to the final area, locate and kill RoBo-PR1M3 (preferably with no witnesses), then instigate Fluffy to attack you in front of other RoBo-B34RS.

It may be easier to trap/injure Fluffy and then piss off the RoBo-B34RS so that they attack you. Then simply stand near the injured Fluffy and dodge the RoBo-B34RS attacks, which will hopefully land on Fluffy until he is killed.

Option to get this achievement if you do not have Gold scores for 6-1,6-2, and 6-3...and have Robo Prime Unlocked yet.
The trick to getting bears to attack each other is to have them accidentally land an attack on a friendly bear.
So with that in mind, Fluffy is armed with a power fist. Best to start with him in my opinion once you get to the final area of a Stage 6 Level, as to not run the risk of him going insane before you get the chance to have him hit one or two of his R0B0-B34Rs.
Just follow him around till he starts swinging at you and either move just out of range or dodge him. Try to stay in his immediate line of sight to keep him interested in attempting to bash your head in, while hopefully attracting a R0B0-B34R or two.
Once you manage to see him punch a R0B0-B34R, you're halfway there, I got lucky, I think, and had him Isolated away from the others, aside from 2 Robots. So i was able to take my time with the next part.
Hit or Trap Fluffy until he starts to limp around, making sure not to mistakenly kill him yourself. Because the R0B0-B34Rs will have a very hard time actually hitting Fluffy even when he is limping around, because of the Robot's VERY slow attack speed.
I ended up positioning Fluffy in-between Myself and the Robots to Stop Fluffy long enough for the R0B0-B34Rs to kill him.
I didn't have the Robo Prime costume unlocked yet, so I was unable to try it how Akito Yuri mentioned above. (His method does sound easier if you have the costume)

You can use the Robot costume gained from getting a gold trophy on level 6 to do this, not just Robo Prime. Destroy the boat and car and injure Fluffy. The Robo Bears will do the rest.