I would argue that "Unwanted Guests" (6th mission, set in Wellspring, featuring Olive) is the easiest.
When you emerge from the basement, there are only two enemies. One has a shield, but you have to throw an EMP grenade to take out the generator anyway.
Next, don't leave the entrance to the bar. Stay at the top of the stairs and take out the enemies in the street one by one. Authority Mutants might charge up the stairs (though only one at a time), but with two shooters that shouldn't be a problem.
When everyone in the street is down, you can either head for the garage and garage's office to pick up goodies, or head to the left and up to deal with two enemies who will spawn when you get near the bend.
Take out the generator at the other end of the alley and deal with 3 or so Enforcers. Head down the alley to pick up goodies, including rocket launchers that you will need near the end.
The next generator is in the street and just down some stairs. When you take out the generator, don't leave the alley. The enemies will all be to the left. Handle this the same way as at the entrance to the bar. Take out the enemies one by one and duck back behind the wall whenever you take damage. Watch for Mutants and perhaps a Shield Trooper to approach. You might consider deploying a turret to distract the Mutant when it comes up the stairs.
When the enemies are done, leave the alley and head up the stairs on the right to pick up goodies. Maybe the designers intended for you to fight the previous battle from the second floor of the house, but if you'd done that you'd have to worry about enemies coming up either of the two stair cases.
Head to the other end of the street towards an open doorway on the right. As you get close, two Authority Mutants will scale the wall on your right.
When you step inside the office, an Authority Elite will spawn outside the other doorway. Blast him with the rocket launcher. You might need more than one clip, but I don't think he will step inside the room you are in.
With the Elite down, go to where the Elite was and use an EMP grenade on the generator at the other end of the courtyard. A few Enforcers will be defending it, but I think that the mission ends as soon as you destroy that generator, regardless of the Enforcers.
I also recommend this mission for the achievement where no one is incapacitated. I've done this mission on normal alone without being incapacitated, and on nightmare with two people and only one incapacitation (the Elite has a lot of firepower).