Where do I get the skill Gladiator's Defense?

I looked everywhere and I don't know what boss to cap it from and where he is.

Gladiator's Defense can be capped at Facet of Strength in the mission The Dragon's Lair, which is accessible after completing ascension in Prophecies.

Here is a wonderful and helpful link to add to your favorites when searching for elite skills.

http://www.xennon.co.uk/eliteskills/

sucks to have to do 5 missions if you're it's a factions char.