Disintegration

If you don't choose Outlaw from the outset, you can replay any level with a new difficulty level upon replay. It does make it easier -- your allies will level up, giving them more health and stronger weapons.

Overall, on Outlaw difficulty there are more enemies, those enemies are stronger, and they are generally smarter. I saw a lot more enemies getting health boosts from other enemies, and more enemies focused their fire on me or on my allies that were low on health.

I completed the achievement on the first level, "Back in the Saddle." Generally it was an easy level, and it helped that my secondary weapon healed my allies. The full gameplay and how to change difficulty settings are in the video below:

If you didn’t play the game on outlaw difficulty anyways, I would recommend to finish the game first on the difficulty of your choice. In that play through you characters will be levels up and gain more strength.

After that you can replay the first mission. Don’t forget to change the difficulty to outlaw before you start.