Extra Credit Achievement
Complete 75 challenges
For this you'll need to focus on finishing the 75 mission-specific challenges, 3 for each mission in the game. The requirements can be found by looking in the challenges section in the main menus. In general, they range rather drastically in difficulty and the amount of time you'll need to spend doing them, so you'll want to look at them early on and try to take them out while you go through your playthroughs and try to get gold on all missions.