Missions are available at various NPCs around the hubworld. Face one so that the yellow carat is over their head and hit
to talk to them.
A new menu will open detailing various mission types. Free Mission is on top. Select that and then chose the mission you wish to work on. The achievements are difficulty specific so make sure you select Easy
. Oddly, they recommend level 10 before attempting them but I had no issues using lvl6 characters.
A cheese method would be to abuse the
If you find one you are good at, you can repeat it over and over for progress on the S Rank achievement.