Medallion's drop from bosses, and the grade of the medallion is dependent on the difficulty.
Although cosmic items do drop on lower difficulties (I personally have seen cosmic gear drop on Heroic and Super Heroic), to get a cosmic medallion to drop on any difficulty less than cosmic is going to be rare. I am not saying it isn't possible (must be possible considering the ratio for this achievement is less than the one for completing the cosmic trial
), but it will be a grind.
My suggestion is to first complete the cosmic trial and get the
Strongest One There Is achievement. This will unlock the cosmic difficulty.
The next step is to basically farm the Daily Bugle op on cosmic difficulty. This op has you fighting six bosses, whereas most of the other ops have you fighting one.
I ran this op 3 times, and got 1 cosmic medallion on each run. I am not saying this is a guarantee drop right out of the gate as this is RNG based, but the odds are certainly more favorable doing this op rather than one with a single boss.
If you are struggling with this solo you can always recruit people from a club to give you a hand (or set up a session here
).
Questions, corrections and neg vote justifications below.
EDIT: As of 11/27, the MHO servers have been taken down and the development team is in the wind. This, as well as all achievements, are now discontinued.