I am going to be starting a mythic campaign soon, starting at level 1, and I am planning on the boss for the first adventure being a level 1 or 2/MR 1 enemy, with the players ascending ,both to MR 1 and level 2, upon it's death. The only problem is I don't know what enemy to have them fight, so I was wondering, what level 1 or 2 enemy could provide a good mythic challenge for level 1 characters, without making it too difficult?
What's your party makeup look like?
Rebuild a BBEG from your adventure path/adventure with the mythic rules. A mythic tier will rise the CR of this BBEG but you can compensate this with a lower class level or the young template.
The easy way is to take one of the simple mythic templates and apply to a CR1 enemy. Or you can build a level 1 pc with 1 mythic rank and use that. Probably a fighter or non-spellcaster so you don't nova the group. Don't take Guardian, players will get miffed if you Sudden Block their crits.
Don't take anything that grants DR either, unless the PCs have mythic weapons (or whatever it is that overcomes the creature's DR). While true at most low levels, at level 1, especially, this could be a game-breaker!