I want to play that category, what do you think is the most optimal epic to start the game with? I was thinking about Prince or Skeleton Horde

You shouldn't need to do anymore than 5 or 6 battles in this category, to reach level 3. I would say that prince is definitely the best, because you'll want to be finishing these battles as soon as possible. Even begginers realise that they can arrow the skeletons, but it takes a few hundred trophies for begginers to realise how to counter prince. But what ever works with you, i guess :D

Baby D and Witch are sure out of question, as their usual playstyle is very low and not adapt for a speedrun. Skarmy might work well with Knight, and Prince with Giant, it's just a matter of rng I guess

For me it's the skeleton horde the best but it's personal.

rage and skeleton army bro