51 Dragons are probably the most serious glass cannons among the DF megabeasts- while they can roast you at range with their breath, if you do get close enough to melee one, their heads are relatively easy to slice off with a good sword or axe blow. Also, they feel pain, so a mere chip to the skull will knock them out.
Hydras are annoying because they have seven heads and will attack you with all of them at once- so you have to decapitate them seven times to kill them. Or just deal enough damage that they bleed out.
Rocs... can fly. However, their melee range is no longer than yours, and you can attack an enemy that's above your head.
And bronze colossi are just tanks. They're made of solid bronze, have omnivision, and feel no fear or pain.
Flukes like this aside, it takes a lot of hits to take one down, even with adamantine weapons.