As most any seasoned Terrarian will tell you, the Destroyer is by far the easiest of the mechanical bosses. On paper, this doesn't look right; at over twice the health of any other Hardmode boss (excluding Duke Fishron), it is lethal in doses over one microgram. Or should be.
I'm here today to explain why it's not.
You see, the Destroyer's souls give us a hint... they're called Souls of Might, implying that the Destroyer has great power and strength. That's exactly where it is. But in having this much power, the Destroyer has little other skills. It relies on minions to cause a lot of damage. It's easy to dodge and can't turn very quickly, if at all; and it has nothing you really need to focus on.
Instead, compare Skeletron Prime. It has four different limbs, each of which moves differently and is quite lethal if left to do its business. A player must keep very alert and active to avoid death against the mech skeleton.
The Destroyer is easily taken down because it can do very little that your mobility won't negate. Its only defense is its massive health pool, but even that is easily sapped away by powerful piercing weapons.
To sum it up: the Destroyer is easier because it focuses solely on causing you damage, but isn't very good at it. It has no unique perks or abilities and is easy to keep an eye on.