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.