I just unlocked this completely accidentally while trying to get it a different way so I'm going to explain an alternative method for those having trouble getting the final hit with an Olog as I've found out that you don't need the final hit to be with an Olog.
1. Find a Captain that has an Olog nearby. Siege Missions and Online Conquests are good for this as Ologs almost always spawn in the final room
2. Attack the Captain until he's fairly low on health. Try not to get him too low or you wont have enough time for the next steps.
3. Apply a Damage over Time (DoT) effect to the Captain such as burning or poison. It helps if he's heavily damaged by these effects to speed up the process of his health draining.
4. Stun the nearby Olog by hitting him in the head with a fully charged arrow. Using the Circle Button also works but I believe you need the Circle Button prompt to stun him that way
5. Mount the Olog by holding R1 and just ride him around. There's no need to hit the captain at this stage but feel free to do so if you're worried he won't die to the DoT effect
As long as you've timed things corrrectly, the trophy will pop as soon as the captain dies to the DoT effect as long as you're still riding the Olog at the time of his death.
I've pulled the Screenshot from my PS4 to show that my trophy popped as soon as the Captain died to the DoT effect. You can see that here: https://prnt.sc/h4mh8q
I was worried I wouldn't get the trophy when I'd accidentally poisoned him so I'm glad I've stumbled across this method and I hope it helps those having trouble with this trophy.