OUTRIDERS

The Final Arbiter is the last boss named "Arbiter of Decent" that you find at the end of Trails of Tarya in a dome shaped arena after beating the Worldslayer Campaign.

Apocalypse tiers are unlocked from the get go, you level up your tier by simply killing enemies on your current highest tier. I was tier 14 when i started Worldslayer. So wherever you stand tier wise, you must get to tier 25/have your game on Apocalypse tier 25(more on that later) then fight your way to the final arbiter boss. Which i will mention you can make a straight B-line to him skipping all the optional rooms.

Now theres really only 2 ways to go about this:
-(The 50-100 hour method)
The only way you'll find this reliable is if and only if you like the game enough to understand how the game works, and have builds that can actually get you through the tiers. This game is very punishing if your build aint set up correctly. So you'll be spending most of the time farming loot and playing whatever activity your comfortable with leveling up. The Devastator class imo is the most OP one. Thats the class most people can solo everything with. But other classes work fine to. Weather your a solo player or like co op play, i say co op is faster and easier for grinding, expect many hours trying to get up to par in order to pass tier 25.
So to sum up, have this be your favorite game for the time being, and grind trails or expeditions on your current apocalypse tier level until your blue in the face and synergize a good build to get you through it.
*As of writing this, for the people who are interested in leveling their tiers up themselves in a cheese fashion, theres currently a method where you kill the final arbiter on your current tier, then you return to lobby immediately after, load back in kill him agian and so on. Im sure theres videos better explaining it but i imagine it'll be patched soon.

-(The Hopping on someone's shoulders method)
Not going to beat around the bush with this, this is the route that most people here will go for. If method 1 aint your cup of tea, then all you simply have to do is load up your save, sit in front of the Trails doorway, pull up the Official Outriders LFG club, you do that by pushing xbox button, go over to your achievements, then youll see looking for group at the bottom, grab that said tea, then sit and wait.
Ive never seen anything like this before, but 9 times out of 10 you'll stumble upon posts from people who are willing to carry you like a loose band aid. You just message them in the post and they will invite you. What i found bazar, and if your lucky enough, they will put it on Apocalypse tier 40! Highest tier currently. Its as simple as that, then you just follow them from room to room and when that person melts the final arbiter your achieve will pop in all its glory. Any loot you find will be at your current highest tier i might add, so dont expect a lvl 75 god roll gjallarhorn or anything.

Its honestly a sight to behold seeing how some players go about playing, especially at that level of tier. Certain hours of the day may seem like the LFG is dead, but its just trial & error. Some days its flooding some days it isnt.

That's about all there is to it, in whichever method worked for you i hope i shed some light on this daunting achievement.

Good luck!