Assassin's Creed Odyssey

To unlock this achievement you need to defeat the leader of any region other than Megaris while they have low resources. Each region has a meter with three bars in it, and every time you act against the leader of a region or their faction in general, this bar depletes. To deplete this bar you need to loot war chests, burn war supplies and kill leaders for the faction that the region is held by. Once you get the meter to the bottom third you will be informed that the regions leader is vulnerable, which is when you can go and defeat them. It's highly likely you will do this achievement without being conscious of it.

Note that some of these region leaders are classed as Legendary characters so I would recommend sneaking up to them and knocking them out, then recruiting them to your ship on the off chance they are a Legendary character. Continue to do this until you have four Legendary characters for the "The Argonauts" achievement, then you're free to kill them as you see fit.