Here is a summary of my findings from my 3-4 unsuccessful attempts and my eventual success in the Test of the Wild.

I completed the achievement with ~14000 points in about 38 minutes using an imperfect version of these tactics. So there should be some extra time cushion if executed accurately.

The main objectives to tackle in order are as follows:
1- Identify and dominate a weak warchief
2- Gain basic intel on all captains
3- Graug rampage
4- Rioting/cleanup

These will be expanded on in greater detail below.

Goal number one is to find a warchief that can be quickly defeated and dominated.

This should ideally be a warchief who is susceptible to combat finishers, though stealth and ranged could also work.

Focus on nearby intel sources, if any. If nothing is nearby, begin grabbing any uruk in the area for intel on captains until you stumble on a captain or a mission is nearby. Use the random captain or the one in the mission for intel on the warchiefs till you find one with an exploitable weakness. Don't forget to kill any branded captain you used for intel.

Now that you have a target warchief, complete their mission to draw them out, brand them, and command them to riot against any other warchief. Take this opportunity to kill or dominate-then-kill his bodyguards as well.

Some would begin completing the riots against the warchiefs immediately but I would actually advise to wait, as I will explain later in the pitfalls section.

Now you need location information on every captain left alive at this point.

Grab any uruks you come across in the world and gain intel until every captain is revealed. Groups of three uruks work well for this as you can quickly brand 2 and grab the remaining one, then regrab your branded uruks for intel as well.

Travelling from small group to group is extra fast if you use shadow strike from your bow where possible, rather than running.

By the end of this, your goal is to have atleast 10 captains that have general locations on the map rather than being in missions. Some may have already been killed by you for intel or as bodyguards, but as long as there are atleast 10 it should be fine.

As a benchmark, I completed this part at around the 8 minute mark with 16 or 17 captains still alive.

Now you get to find all those captains roaming the world and eat them with a Graug.

If you need to know where to find Graug bait, this guide shows the locations of the bait in the world.
https://gtm.you1.cn/sharedfiles/filedetails/?id=517999970

I would suggest beginning the massacre just outside the bridge to the fort in the upper right corner of the map. Travel counterclockwise around the map from area to area eating every captain, saving the fort in the upper right for last. Only move to the next area when all captains in the current area are dead.

Roam the areas with wraith vision looking around till you see the option to focus on a captain. The captains seem to spawn somewhat randomly. If you are in an area that is supposed to have a captain and you can't find them, run around in big circles a bit and try to cover every part of the green area till they spawn in. This should still be faster than having to come back to find them later.

The forts in the upper left and bottom of the map are mostly navigable by the graug so it shouldn't be too hard to reach any captains in those areas, but we saved the worst fort (upper right) for last.

The bridge and first courtyard of the last fort are accessible to the Graug, but the last two areas cannot be reached as far as I know. 2-3 captains usually spawn in the back areas of the fort that the graug cannot reach. Hopefully, by the time you hit this area you have already achieved 12 or so monster kills. If so, feel free to dismount and just kill these last captains by hand. Or if they are bodyguards for warchiefs you could choose to leave them for rioting.

If you reach the wall and somehow do not have 10 monster kills, it is possible to dismount the graug, draw aggro of nearby captains, and return to your graug to eat them when they come chase you. Just don't let your graug die if you are trying this. Watch its life and return when necessary to remount and eat a few uruks to regain its health.

Note that the graug swings his left arm to eat and usually grabs the first target he hits so try to keep your left side relatively clear when aiming to eat a captain, or just spam EAT until he's all that's left.

By now you should have 4 non-branded warchiefs, 10+ monster kills, and a handful of captains alive.

The remaining captains should either be in missions or be bodyguards for the warchiefs.
Complete the remaining missions for any captains that are not bodyguards until only warchiefs and bodyguards remain.

Now we riot.

Complete the riot you initiated in the first phase. Grab your warchief and command him to do the next riot and repeat this process on all 4 warchiefs.

Try to utilize any of their weaknesses to speed up the riot (i.e. vulnerable to ranged, stealth, combat finishers). Do not allow any bodyguards to escape if possible, they will become the next warchief and you will have to complete another riot for them if they do.

If you have a warchief to fight who is immune to ranged, stealth, and combat finishers there are some ways to help even if he is hard to damage. You could find nearby graug bait if there is one and bring a graug to eat him, or you could kill all his followers and repeatedly stun him while allowing your allies to hit him.

At the end of the last riot you will have only your branded warchief left alive. Dispatch him and the test ends.

You need to end with at least 12000 points for the 1000 bonus for speed to bring you to 13000. So make sure you are in the ballpark before killing your warchief. Before dispatching mine I was at around 11700 points and ended slightly above 14000 to give you an idea.

If you have time, and worry you aren't close enough, you can kill some enemies until you feel you are within range and then attend the "beck and call" mission to dispatch your warchief later. Otherwise, here are some tactics to help scrounge some points throughout.

-Kill every uruk used for intel. Just a few extra points for killing them rather than dismissing them.
-Run through slave/uruk groups during the rampage. This should liberate slaves and gain those points.
-Kill your graugs when done using them.
-When your warchief despawns, after commanding the next riot, dispatch all the followers he brought to the riot.
-Kill as many of the uruks the other warchiefs bring to riots.

Here are a couple things that can really throw off an otherwise good run.

Dying-
Dying causes lots of things to happen and can promote new uruks, have captains killed in duels, etc. as well as generally waste precious time. Just don't die if possible.

Repeat Fights-
Try to make every encounter with a captain/warchief a one-time event. If everyone you encounter dies during your first meeting it will save time completing additional missions/riots or tracking them down again.

Long fights-
If you happen to find a captain immune to everything, it may be worth it to abandon a 5-minute fight with them and save them to be eaten by the graug during the rampage.

Late Monster Kills-
There seem to always be 2-3 missions for captains on the map. These captains are therefore not spawning in the world. If you kill too many captains by hand before reaching 10 monster kills it is possible that the only remaining captains will be in missions and not available for your graug to eat. If the missions they are doing don't have monsters or nearby bait it may be impossible or very time consuming to try to kill them with monsters.

Rioting Immediately-
Rioting immediately can help identify and kill many captains as they are brought to the riots as bodyguards. However, any that escape will become the next warchief and force you to complete another riot. Additionally, killing all of the bodyguards and warchiefs in riots, in order to prevent a promotion, may cause the "Late Monster Kills" problem mentioned above. Completing 1 or 2 riots right away could be a good mix of speed without killing so many bodyguards that you can't eat 10 captains with the graug, though this was not the tactic I used.

For all other achievements I used this 100% guide to help me unlock them all.
https://gtm.you1.cn/sharedfiles/filedetails/?id=2066155450

Best of luck.