The Assassin's Creed one, Lightning Bahamut Battle DLC, Mass Effect were the most downloaded content of them all. The Mass Effect one was pointless and were simply ugly when worn on them. Can't hate on it though for being purchased the most, people have different tastes out there.
It's not hard if you debuff, there will be a live trigger event when you RAV one of them to 999%. Ultros is 100% crystal. Typhon I believe is 20%. The hardest one is Valfodr and Gilgamesh.
Valfodr is hard because he strong resistance to debuffs along with different levels of him. (Valfodr 70 and Valfodr 99 are the most painful.) It's not even a challenge battle, it's a silly battle of endurance and keeping stamina. Valfodr has access to Slowga, Ultima, and Cursega on the first phase. Then it becomes a massive problem when his health drops to half. He will cast Deprotega and use an attack called Gagnrath and Quasar. (Quasar I believe causes heavy wound damage, which can prevent certain health to coming back.) and the best of all to top it off to make the battle longer than it has to be is that he can buff himself with Haste, Faith and Bravery making the battle less fun.
Gilgamesh has 999,999 HP. The first phase is easy because he can get poisoned, but the 2nd phase is the hardest of all the boss fights within that content. Gilgamesh has access to Launch and Blitz. (Anything that launches you is the best attack in the game because you are forced to take damage, evasion from SEN has been reduced to 0% in the air, you cannot commence a turn until you have fallen back into the ground and awaiting the recovery off of it which is the worst when you are casting a buff like Haste with moves like this eating up the buff time.), you know that animation when you are launched by the Meteor attack from PuPu for attacking the ship? That. This is one of the tools that drastically makes the game easier when you stagger them into 999% with you constantly launching them and doing massive damage to them. When Gilgamesh is half health, he will cast a permanent Haste on himself that can't be removed along with increased stats of Strength and Magic which allows himself to attack in situations where you are launched or trying to attack him. He can literally do Blitz > Launch > Cross-Slash > Death Claw > Blitz while you are literally *still* in the air, you are not allowed to heal while you are in the air either. Challenging at first, but it got really annoying when he got a permanent haste.
Then the worse of it all is when you beat them (Valfodr) and you don't get the crystal even though it was a 5-star battle reward so you don't feel like doing it again.
@Awesmic may say the same thing or probably fill you in more about it.
