The Conquistador Elmo is actually a Lancer/Dragoon "Medium Armor" piece and not a Tank "Heavy Armor" piece like the Steel Elmo.

Tank "Heavy Armor" pieces have "very high" Defense and Magic Defense for their ilevel.
Dragoon "Medium Armor" pieces have "high" Defense and "average" Magic Defense for their ilevel. Prior to the 2.45 Lancer/Dragoon buff, Medium Armor had "low" Magic Defense.
DoW Light Armor has "average" Defense and Magic Defense for their ilevel.
DoM Robes have "low" Defense and "high" Magic Defense for their ilevel.

Lancers/Dragoons have access to a few tank sets. These sets are the Iron Scale (23), Steel Scale(32), Horn Scale(38) and Tortoiseshell Scale(46) sets. The Steel Elmo is part of the Steel Scale set.