Fortitude runeword

Fortitude can be made in both weapons and armor. Because of the high cost of a Lo rune, and many better alternatives in the weapon slot for that price, Fortitude is usually only made in the armor slot. The best armor to make Fortitude in is a high-defense Archon Plate. For a mercenary, the best armor should be ethereal and ideally be superior. If you have a low level mercenary, it's best to also go for an Archon Plate. If you have a high level mercenary (lvl 90+), you can opt for a Sacred Armor, given the high strength requirement.

Because of its high Enhanced Damage, Fortitude is most useful for physical damage characters like Smiter Paladin, Bowazon, Kicksin, or Frenzy Barb. When created in an ethereal base, this is also a perfect armor for any physical damage mercenary.

Enhanced Damage on an armor does in fact work and increases physical damage. First all weapon damage modifiers are applied, secondly all offhand damage modifiers are applied. This means that if you have f.e. +200% Enhanced Damage (damage x 3) on your weapon and +300% Enhanced Damage (damage x 4) from Fortitude, you will multiply your damage by 12 in total.

On top of this ability to massively enhance physical damage, it also provides many benefits to survivability like Enhanced Defense and All Resistances. However you should be careful when selecting this armor for defensive purposes, as Chains of Honor could be a better alternative in some cases.