Naughty Bear

For this achievement you either need to kill, or POP! a Unibear. Their placement seems random at best, and they disappear very fast after appearing.
Added to this, you don't get any real warning when they appear, but you do get a chime sounding and the camera screen will pop up in the corner of your screen stating that the Unibear has appeared.

I got mine whilst running through mission 1 challenges, and it just appeared right in front of me so I'm guessing luck will be a factor.

As far as making them kill themselves, just attacking it once seemed to make the bubbles appear so go for that if its your bag.

NOTE: It has been mentioned that if you delete your updates and play the game offline, you are more likely to encounter a glitched Unibear who doesn't despawn. I have personally had no luck getting this to work, but if you get desperate it's worth a shot I guess.

In each area you will come across a unibear statue. As tempting as it is to break it for the "broke a pretty thing bonus", keep it intact. These seem to up the chance of a unibear appearing considerably, as do some insane bears. However, be aware that any signs of chaos mean the unibear will instantly disappear. I tried the statue tactic and got a unibear first time. Since then it has appeared another 4 times so whilst not a guarantee this was the only was I could get it to appear and seems a solid way to get it to appear.

Make sure you are wearing a costume that does not draw attention to yourself so you can approach it. I wore the Cop Gordon costume. Popping the unibear occurs when you make it commit suicide, or you can go for a straight superkill. Do not attempt normal combat as it will be off before you have a chance. If you kill it in public this will really disgust the other bears so it is good for insanity challenges and points challenges. Happy unibear hunting!

Secret achievement= kill or pop a unibear.
First let me say the other guides failed for me so I'm writting up how I got it.

I got every other achievement in the game besides the 250 million points and I never killed or popped the unibear.

It turns out that if you delete the update you have a MUCH better chance to get a glitched unibear or a unibear that appears infront of you. So after I deleted the update (clear cache, harddrive settings) I loaded up level 2 (normal mode, not challenges) and selected the last available bear with the grey hat. I then ran over to the house broke the power box on it, then ran to the car and sabotaged it and then ran into the woods near the car and looked at the firepit. 3 minutes later after just sitting there the unibear appeared and he was glitched.... I could stand infront of him and he didn't disappear. I hit him twice with my bear fist and then I got the prompt to kill

Achievement unlocked. Good luck, this is worth the try if all else fails.