Crimson Dragon

As far as I can tell, mission medals can only be earned on classic or insane difficulty. If you have trouble on classic, drop down to casual and level up your dragons. You can view medals by pressing for "more info." Here's some easy ones:

Coral lake East: The Draco connection: Complete without a wingman. Wingman don't add much firepower, so just unequip yours.

Coral lake West: Holding the line: Earn a kill rate of 70% or better. Requip your wingman and kill stuff.

Underground Lake: The subterranean Outbreak: Kill rate of 50% or better. Easy

Underground Lake: Stemming the Tide: Complete without a wingman. There's more "complete without a wingman" medals, but these are the easiest two.

Lava Lake: The Lichfield: Complete without a wingman, Score 50,000 points, or Get rank B or better. All of these are easy if you use Slivertail. Slivertail's basic attack kills the lich in seconds, earning you an easy S. Score and rank are based almost completely on the time it takes you to kill the lich. A wingman wouldn't have time to help at all.

I earned one Medal naturally by trying to do the "Skilled Seeker" Achievement.