Minecraft Dungeons

The easiest and fastest way to do this is on the cow level (???), if you have it unlocked. If not, just look up how to unlock it, literally takes less then 30 minutes with most of the levels unlocked and it's great for leveling up. You could just use a friend that has it unlocked also.

The Raid Captains are tall human-like enemies with a banner above their head, a "boss bar" will appear at the top when your close to one. They appear on a level after you have completed it at least once. Simply kill it and you get a modifier added to your game, they are random, some of mine were "Threat Increased" and "Mob damage is increased by 50%". Then just finish the level, the cow level should take no more than 5 minutes to finish, so your looking around an hour for the achievement. You get an extra chest at the end for finishing with a modifier.

Difficulty doesn't matter and killing more than one captain to get more modifiers will not double your completion percentage. If playing with more then one person, everybody in the game gets percentage.

Guaranteed Spawn: Pumpkin Pastures

After you finish Pumpkin Pastures for the first time, Raid Captains will always spawn in the level. The easiest way to find one is to head to the pirate ship that is located at about 1/3 of your way through the level. The majority of the time you will be required to walk through the pirate ship to continue the mission, but sometimes it will be off on your right when you are walking along a cliff and you see it down below in the harbor. Just move forward and then around and to your right to get to it in that situation.

On the pirate ship is a door you can enter to get to a secret room with a supply chest. There is always a Raid Captain within this room. Just kill him and take his banner then finish the mission and you will get progress towards the achievement.

If you lose too much health fighting him, just leave the room and wait outside until your potion refills or use a healing artifact, then go back inside to finish him off (his health doesn't regenerate).

There are also multiple Raid Captains throughout this level off in the side paths (I usually find 3-4) but the pirate ship is the best method since it's on the way. Don't bother picking up additional banners as they don't increase your progression (only one per mission counts).

If you want to get this done as fast as possible then just put the difficulty down as much as you want and run through the level. Although it's maybe not as fast as the other solution (should take 10 minutes or so), it's a guaranteed location, so there's peace of mind in that.

Note that the Pumpkin Pastures Daily Mission, when it shows up, does NOT have a Raid Captain.