You get this achievement by killing Mercenaries. There are nine tiers in the Mercenaries tab, and the object is to get to the first tier any means necessary. You only move up one rank no matter what. Even if you defeat a mercenary two or more ranks above you, you still only move up one rank. You could kill the first ranked merc if you wanted but you would still move up one rank within your own tier. If you're at the highest tier in your current rank, you would move to the that tier to the lowest rank in the next tier. You do NOT move up if you kill a lower ranked merc.
Each tier has a bonus, such as discounts with the blacksmith, and tier 4 has 20 percent off for ship upgrades.
If you need to move up but can't seem to find a merc above you, the arena usually has a merc for you to fight. To get there, fight a couple mercs, and you will get a quest. Then sail to Pephka on the bottom right corner of the map. Tell Skoura you want to fight the merc (bottom option). If you win, you unlock:
The Blood Sport achievement in Assassin's Creed Odyssey worth 31 pointsDefeat a Mercenary in the Arena.
The bounty system can also help you move up throughout the Mercenary system since it provides a constant stream of mercenaries coming at you instead of needing to search for them. However, if you're level 5 on the bounty scale, you may end up fighting two or three mercs, so if may be too difficult to win.
You can also recruit them as crew members. You can use your eagle to hover over them and scan them. If the mercenaries has a gold background on his or her thumbnail, go ahead and Sparta Kick them (wait until their health is low and there are no cliffs). Kick to end the fight. Then, press while standing over the body.
If you recruit four legendary lieutenants, you'll unlock:
The The Argonauts achievement in Assassin's Creed Odyssey worth 46 pointsFully crew the Adrestia with Legendary Lieutenants.
Anyway, the mercenaries seem fairly impossible at first, but as you build your character, they get easier.
First, make sure you're upgrading your spear. Check out TheDude722's Legacy Restored guide on how to do that. You want your spear to be level 5 by the time you're in tier 3.
Next, here's the build I used:
Stealth Master: 3/3
Critical Assassination: 3/3
Second Wind: 3/3
Flaming Attacks: 3/3
Weapons Master 3/3
Gear Master: 3/3
Overpower Attacks: 3/3
Shadow Assassin: 3/3
Fire Mastery: 3/3
Battlecry of Aries: 3/3
Sparta Kick: 3/3
I didn't mention abilities I didn't use for the fights.
You also want good weapons that can produce high DPS.
Another thing is overpower attacks use up three adrenaline points. So, with your spear at level 4, you can perform two overpower attacks if your adrenaline is maxed out. However, if you defeat the Minotaur, you will get Minotaur's Labrys, which gives you the "-1 Adrenaline Cost for Overpower abilities" engraving. Go to any blacksmith and have him engrave any weapon you want. You can also do this for as many weapons as you want, though only one engraving can be used on any weapon. Now you can perform three overpower attacks in quick succession with a full adrenaline meter. You should also add assassin damage to your gear.
I typically only fought them if they were one or two levels above me. Oftentimes, killing a merc that's a higher rank would reveal another merc, so you don't really need to keep your wanted level up.
Anyway, here's how a typical mercenary fight went for me:
First, make sure you're out of combat and you don't have a bounty on your head (if you have one, go you your map and hold ). Now, hold (Critical Assassination) to get his/her health low, then Flaming Attacks (with Fire Mastery), followed by overpower until you're out of adrenaline. Finally, finish of any remaining health the merc has and any animal companion.
Also, if you're using the Bounty System, you can kill them easily with the Adrestia (Check out PsYcHoZeRoSk8eR's Lord of the Seas guide for tips on upgrading it). Ramming at full charge (hold from a couple hundred meters away) can do the most damage if done properly.
The top tiered mercenary (level 50) is a cultist so if you're going for the cultist achievement, you will want to go for this one as well. I killed him at level 47, and I wouldn't recommend fighting him until you're at least level 46. Anything below that is a one-hit kill for him.
However, according to comments, you can kill the cultist merc without getting the achievement. People have found and killed him while they were on tier 6. You do not move all the way top the top by killing him.
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Finally, if you give me a negative vote, let me know why in the comments section so I can improve my solution.
(Special thanks to PBreezy for helping me with the organization of this guide.)