It depends heavily on which versions of the myths and stories you are using as reference.
Some versions of Arthurian myth paint Merlin as just a very wise man, others have him being the son of Satan who would have been the Antichrist if he had not been baptized.
Conan is more of a fighter/rogue than a barbarian(berserker), but his carrier is easily mimicked by every edition of D&D.
Heracles/Hercules is a victim(or more correctly benefactor) of power inflation. Demigod basically is Greek term for hero, and as possibly one of the oldest Greek heroes his accomplishment have probably been embellished the most. If you strip out the embellishments, the Labors are all things low to mid level fighters could potentially do: Kill a lion, large serpent and flock of predatory birds, capture a deer, boar, bull and ferocious dog/wolf, single handily clean out a dirty stables, seduce the female leader of a rival tribe and bring back proof, steal herds of horses and cattle, and steal apples from a well defended orchard by bribing/tricking one of the workers.