Assassin's Creed Odyssey

To unlock this achievement you need to raise your bounty to the highest level. There are five levels of bounty and to raise it you need to kill soldiers in open combat (stealthy kills won't raise this as you arent seen killing), steal in the open and generally cause mayhem. This can be done at sea too by attacking enemy/mercenary ships. As your bounty increases more and more mercenaries start to come after you so it can get tough, especially since sometimes the mercenaries in the area may be higher level than you and attack you more than one at a time.

A good way to get your bounty up is to go into an enemy encampment that has a brazier alarm and get spotted, then let someone light that brazier to get reinforcements called. Try not to kill everyone too soon and before long someone will go again to the brazier to light it a second time to call in more reinforcements. Doing this i ended up getting attacked by four mercenaries at once, two of which were one level above me and 1 that was three levels above me so it proved very difficult to survive, so do be careful. You can also set the Brazier off yourself by shooting a fire arrow or throwing a torch at it.

Your will also get a large increase to your bounty if you kill the Leader of a region out in the open in front of witnesses, so you could also try this for a boost, but beware of the guards surrounding a leader once you kill them as they are usually very strong.

This achievement can help with progress towards the "Top of the Food Chain" achievement as you need to defeat mercenaries to make your way through the ranks and become the top one, so do be conscious of this too.