Boss difficulty idea, other than damage atenuation

[AreeSoothsayer]

You have the shield guard, where shields will hit 0 and ignore the rest of the damage.

You can give the bosses, Boss Armored Health. Instead of a single bar of health, it would be split up. Blocks of health. The end of the Block would do what shield guard does. Stopping all extra damage and giving 1-2s immunity to incoming damage.

The bigger and badder the boss, the more Blocks of health the boss would have. This is independent of overall health and level. For example, the Sergent would be a 1 block health. No armor at all. Lephant, each of the heads would have 3 Armored health. When a bar of health reaches 0 it could trigger a reaction. Liches, Ladies, they could incorperate the Armored health since they are like a sub boss.

To be really evil, you could make a Window, where a certain type of damage has to hit the Wall in the armored health or it would regenerate to fill That bar of health.