A side-note to getting the medals necessary to unlock this achievement:
A way to make it much easier to get through a level 12 explore is to have a team comprised of troops that can convert gems of a given color to skulls or doomskulls.
If you have Kurandara, this becomes much easier to do. It doesn't necessarily speed up getting the souls needed but in terms of getting the tokens, upgraded through badges and finally medals, this becomes much easier to do.
The team I use is as follows:
Glaycion (turns red gems to doomskulls)
Kurandara (turns yellow gems to doomskulls)
Moira Cragheart (turns green gems to doomskulls)
Forest Guardian (turns purple gems to regular skulls)
This method works wonders in getting through high-level explore runs. I've been able to get through more than half of all level 12 explore runs with this team as the doomskulls unleash devastation on the enemy team. If you can get multiple 4-skull matchups, the damage is devastating. Sometimes, you might even clear over 90% of the entire board just from multiple 4-plus-doomskull matches. This has often gotten me through many level 12 explore battles.
If level 12 is still too much, you can still at least play explore runs on the highest difficult you feel you can handle (I usually play at level 10) and earn enough mythstones to play and beat the mythic boss battles to get the medals necessary.
However, as RadicalSniper99 says earlier, this doesn't guarantee a 100% chance at winning. You'll occasionally find yourself in situations where the gems and skulls just don't line up right. Be prepared for the rare but infuriating case where a mythical boss battle is lost because of a bad lineup of gems skulls.
If you don't have Kurandara, you can use any other combination of troops that converts gems to skulls or doomskulls.