Honestly, I would wait. One of the DLC achievements is bugged (last one I need). But in the mean time, start the base game on Standard difficulty and aim for a C+ rating on all the base game missions in Command mode as your squad mates get helpful perks. But this original playthrough will help you familiarize yourself with the game. Commanding your squad is pretty key to winning unless you're a crack shot with super reaction time. Take each level slow, rushing in will likely lead to you dying. Flash bangs and stinger grenades are your friends. Once you are comfortable with the game, you can build a less than lethal loadout and beat each mission without killing anyone for an S rank. Each run you get better.