When you click on "Solo Missions" from the main menu, select a mission, and when you're in the mission briefing screen, which lists your score, leaderboard info, objectives, mission type, length, etc., look at the left side. You see 2 tabs, "Ranked" and "Dynamic".
You have to touch the "Dynamic" tab. When you do, you notice the objectives change to "Objectives in dynamic missions are randomly determined." You'll also see your score is different (score will be 0 your first time doing a dynamic for that specific mission). Accept the mission and then complete it. It doesn't matter what your score is when you finish it, as long as you do. If you die, just keep at it until you win.
Personally, I just did the first 13 missions on dynamic, but if you have a preference for missions, feel free to do those. Just make sure you do 13 different missions, as doing the same one 13 times (even though dynamic makes each time unique) will not net you the trophy.
Happy grinding