Quote Originally Posted by Squall of SeeD
The Dark Knights should have stuff that will increase their defense and try to get their HP as close to 9,999 as possible. To do this, I used a Mythril Bangle with a Champion Belt (Defense + 40) on one, and a Crystal Bangle with a Crystal Glove (Defense + 60) on the other. I also placed the Valiant Lustre Garment Grid on them both (Defense + 20).
Or, better yet, you can unlock the dungeon inside the Chocobo Ranch and fetch the Higher Power Garment Grid (Break HP Limit) for your Dark Knights. Then equip each of them with Crystal Bangles, one of them with the Key to Success (in Tobli's house in Guadosalam during Chapter 5; fully complete Moonflow missions, but don't stop Garik on Gagazet during Chapter 3), and the other with a Champion Belt. That worked for me, nicely.

Note: I skipped the Via Infinito the first time through so when I faced Trema for the first time, I was able to replace the Champion Belt with a second Key to Success.