Assassin's Creed Odyssey

To unlock this achievement you simply need to reach level 50. This is self explanatory, but you get xp from killing enemies, completing quests and finding locations, among various other activities. You are not likely to reach this level until near the very end of the game or even after completing the game so i wouldn't worry about going for this consciously and let it unlock naturally. You can buy a 50% xp boost from the store to speed this up, but it's not necessary. I did a lot of side missions and exploring as i was playing the game and got to level 46 without any grinding.

I had to grind the last 4 levels out but there are a few decent quests, such as the two 5-stage quests that have you deal with the Polemarchs of each respective faction. You can find Demosthenes in Athens who will give you the quest/s to kill 3 Spartan Polemarch's then loot a seal from them and return it to him. You get to repeat this 5 times for decent xp. You can then get the antithesis of this from Lysander in Sparta who tasks you with killing 3 Athenian Polemarchs and again you can repeat it 5 times for a lot of xp. You can also fight perpetually in the Arena once you have beaten the champion. 5 more level 50 opponents appear who you can fight for xp as well as any bounty hunters/mercenaries that may be after you.