I would only add the Hydra if there was a swamp race in the expansion pack; it would be much more appropriate there than among the dark elves. But I would replace it with an archer, yes.

By the way, I am extremely shocked by the discrediting of MINOTAURS in duel mode – they are not available to any hero!!! This is outrageous! I forgave the developers for severely weakening these units, but why humiliate them like this??? After all, no other race has units, not even a single, unupgraded or upgraded form, that are not used in duels at all 0_0.