This achievement will take some time, but will also help you complete "10 Items No Less" and "Xenocide". In order for you to get this, all medals must be earned on the same type (I stayed on open) and difficulty (I did easy). Every medal must be earned on a different mode (story, grinder, practice, etc.) for them to count towards the achievement. I recommend not even bothering with split screen or multiplayer (except for achievements containing those modes) as those medals also don't count I believe. The prologue also does not count.
I would recommend starting by either completing every mission on practice or story, and then doing it on the other one after completing on of those. This will earn you 40 of your 100 medals straight away. (Practice because it's the easiest mode) After this, I'd recommend going back to missions you liked/found easy and doing every mode (except the multiplayer ones) on them. The mission I found easy were 1, 2, 3, 4, 11, 13, 14, 17, 18 and 19. If you would like to do this achievement the fastest, then the first missions are the way to go because they have less waves. If you would like to complete "10 Items No Less" as fast as possible while still making progress on this achievement, I'd recommend doing a lot of grinder/super grinders because from my experience they give you items more often.
You can check your progress on the game hub. The only reason you shouldn't get the achievement exactly on 100 is because you got a gold medal on a mode in a mission twice.