Hmm, the question is actually quite tough in my opinion. Each unit has another than can be the death of them. Archdemon beats the black and red dragons easily. Against a green, they should still win due to their high initiative (8 vs. 5 for the green dragon), high move speed(9 vs.6 for the green dragon) and higher physical resistance (20 vs. 10 for the Green Dragon). In general, the Archdemon can get 2 free hits against the green dragon before suffering any hits (move away, wait, green moves, archie hits, next turn, archie hits) not to mention that he can outrun the green dragon.
Against a bone dragon, things are interesting. Archdemons once again have higher move speed (9 vs 7), higher initiative (8 vs 6), higher attack and defence (66 both vs. 53 both), and higher crit chance (15% vs 10%). HP's are 766 vs. 790 in favor of the Bone Dragon. Damage is 88-99 vs. 50-80 in favor of the Archdemon, but the bone dragons have 10% physical resistance, while the Archdemons don't have anything to show for against the poison damage.
However, pure math and 2 free hits put the archdemons slightly ahead, if I'm not mistaken. So, in the end, the Archdemon should take the grand prize
