Based on the circumstances, it’s impossible to achieve a Mythic Medal in Halo Infinite against enemy players. I mean it’s possible to achieve it in Halo 5: Guardians if playing Grifball or S.W.A.T.
https://www.halopedia.org/List_of_Halo_Infinite_medals
The Ninja and Remote Detonation medals might be one way of doing it.
Read the link. Mythic medals are a category of metals. Beyond killing 15 enemies in a row without dying or getting an overkill these are the easiest to get.