Mythic medals refer to a category of medals (all of which are broadcasted in the kill feed when achieved) that are distinctly recognizable by their red color palette.
Mythic medals include:
Overkill: Kill 4 enemies in rapid succession. (and any multikill medal above)
Running Riot: Kill 15 enemies without dying. (and any spree medals above)
Ninja: Kill an enemy by jumping over them and backsmacking them.
Fastball: Kill an enemy with the impact from a thrown grenade (basically, kill an enemy by hitting them with a grenade, NOT by the grenade's explosion)
Perfection: Win a game with 15 or more kills and no deaths.
Quigley: Kill 2 or more enemies with a single round from the S7 Sniper.
Remote Detonation: Kill an enemy by shooting a grenade.
The easiest way as of right now is to use the Bot Bootcamp matchmaking playlist, which pits 4 human players vs 4 bots. Do not confuse this with Bot Training! I found the easiest thing to do is go for Ninja medals as they only require a single kill and is relatively simple to pull off with bots.
Keep in mind 343i might patch this method out as the achievement is obviously supposed to be done against human players, hence the "in a matchmade game" description and the high gamerscore.