Experienced

Play 5 Outbreak matches.

Another self-explanatory trophy: simply play Outbreak 5 times; 5 matches against the AI will do the job, and you don't have to win them or even accomplish a single thing in them - the match simply needs to be completed in order for your progress to be saved.

I will point out, though, that any new items you may be rewarded with in Outbreak can be used in the Story mode of the game, which might prove helpful to anyone looking for a bit of extra help going into a Hard difficulty playthrough.