Too Human

There's a little bit of confusion surrounding this achievement, after trying the various different methods posted around the internet I finally found out for certain what you need to do.

As stated in the achievement description one player needs to destroy the Trolls chest armour while the other player needs to mount the troll and get the mount kill.

Now, the trick to getting this achievement to unlock is whoever is hosting the game has to be the player who performs the mount kill, he shouldn't inflict any damage on the troll at all, just to be safe.

Before the host can perform the mount kill, player 2 has to destroy the chest armour, use which ever tactics work best for you, personally I used my rifle and just kept firing at him until I'd destroyed the chest armour, while attacking a troll it's shield is represented by a yellow bar, once that's depleted it will turn into a green bar, this is health bar, you can take the chest health bar out too if you want, so long as the yellow bar has been depleted you're done on this end.

Once you're ready for the host to perform the mount kill have him move in on the troll, it's easier if you can approach from behind but it isn't essential, when you get close enough to the troll (you do need to be behind him to initiate this) a small "A" prompt will appear on screen (for the host), press the "A" button and this will initiate the mount kill, now you'll be riding the troll, he'll try to shake you off a bit so you need to use the "LS" to get your balance, once you know what you're doing here this bit is really easy and if you've performed a mount kill before then you should already be familiar with what you need to do! Once you've gained your balance use the "RS" to perform mount kill, as soon as the short animation is finished the achievement will unlock for both players provided you followed these steps.

A quick recap;
• The person performing the mount kill must be the host!
• The person performing the mount kill should avoid damaging the troll in any way (apart from the mount kill)
• Upon successfully using this method the achievement will unlock for both participants!

Online Co-op.. you take off armour of troll, and have a buddie jump on his back and make the killing blow, or vise-versa

and gg. 15t/10n GS

If you have trouble getting the achievement then use a new character and get your team mate to do the same
One of you destroy the armor and nothing else. the other needs to run behind the troll and press A
Then ride the thing left and right, pressing down when even to finish the troll off

I have made a video which takes all the solutions posted so far into consideration. Me and https://www.trueachievements.com/reb31.htm attempted this in a recent co-op session and managed to unlock the achievement on the third attempt. With reb being the lower levelled character, he hosted the co-op game on Ice Forest 2-1 and there was an Ice troll to attempt the achievement on right at the beginning after a ton of smaller enemies attacked.

1st attempt - Failed possibly because I stripped too much armour off of the troll.

2nd attempt - reb hit the troll once by mistake.

3rd attempt - reb didn't touch the troll at all and I stripped it of just its chest armour before reb mounted and successfully killed the troll. Achievement unlocked!

Here's a quick video of how it went down:

One Player Destroys all the Armor (YELLOW BAR),this contains Foot,Arms and Body with your Guns.And then the Second Player goes behind the Troll and Jumps on the Troll with Button A, then balance his Character to the middle and push the right Stick down to give the Troll a instant Death.
Achievement unlocks for both.

When I got this achievement, I was the host, and removed the armour, while my buddy mount killed the troll. If anyone would like help with this achievement, get in contact and I will gladly help out and with any others that I can.