Previous solutions did not help, even tried for hours… So got mine. :) This WAS my last achievement. I finished the game, I am on the max level 60, but you can use the method earlier, just at the end, it is very easy with this. The achievement requires a captain, but NOT enemy. This is important.
In short: Mount an olog, while one of your follower captain bleed out, so dies.
1. You must resurrect a captain - Y on dead follower in ARMY screen, level doesn’t matter. Important to resurrect, because the undead Captain can not betrayal, if attack him. Make him bodyguard.
2. Change your bow to a low one. So if you missed the shot you can try immediately, because don’t make much damage and/or fire/poison effect which avoid to jump/ride the Olog. I used a level 18 Legendary.
3. Go to Gorogoth Fortress – maybe other fortresses good as well or somewhere you can find an Olog. In the fortress no one will attcak you (IF THE FORTRESS IS YOURS!), so easier. Find a chained Olog, there are many (red icons on the small map, left bottom). Tip: kill your followers near it, if want (skill tree bottom right skill „Destroy followers” to explode heads), so they won’t attack the olog upon release and won’t disturb You to aim/jump on it.
4. Summon the bodyguard and hit until bleeding (won’t betrayal :) ). The bleeding time is 60 secs. The „Ride on the Olog” is 10 secs (+ release and aim time). So when less than a quarter left of the bleeding (~10 secs the best, so you have time for 2nd shot if 1st missed/can not jump on Olog), release the Olog (RB next to him), aim, shoot his head with a fully charged arrow and hit RB to mount the olog. For this, you MUST stand near him! If you are mount the olog, don’t have to go anywhere and don’t have to be near the Bleeding Bodyguard, because he will die by all means, and if one of your followers bleed out the system will shows you, even you are on the other side of the world. :) So just wait for the achievement. Achievement pop.
Got it my second try! (On first try my level 62 arrow made fire effect and the olog pushed away, so can not get on him)
If somehow you missed it, just start over, the undead can be ressurrect over and over again for free. I recommend to try this just later in the game or at the end.