(for testing purposes, I'm starting my sixpence on this subject)
55 is a reference to this builds unusually low hitpoints, namely exactly 55 hitpoints.
The skill "Protective Spirit" reduces received damage to a percentage of your max hitpoints, so reducing your hitpoints lets you receive very little damage, from any source, being it physical or elemental. Using a regeneration or shielding skill, it turns your character virtually invulnerable to incoming damage.
To reduce your hitpoints to 55, you require a *Superior Rune* on every armor piece. The 5th Superior rune is double, you will not receive the bonus on your attribute, but he -75 HP of a Superior Rune will stack, giving you 105 hitpoints. Furthermore you require the "Grim Cesta" in your offhand, an item which is a quest reward from the "Cities of Ascalon" quest in the Prophecies Campaign, has a -50 HP modifier, finally resulting in the necessary 55 hitpoints.
To receive back energy (and adrenaline), usually "Balthazars Spirit" and/or "Essence Bond" is used as a maintained enchantment. You will receive energy each time you are hit.