The difficulty of Asch depends really on your lvl and which Artes you are most intune with. For me it was relatively easy since I knew how to dodge and everything and I had the ultimate sword for him, not the sword in the video. I also had nothing but Arcane Artes equiped so that I could use my fav arte and the my Mystic Arte. Guardian Field was the one that I used most often since it surrounds Luke in an attack spell and hits multiple times.
Thanks to the random increase in HP for Luke, I'd guess that he's in his lvl 60s. It's a little difficult at that lvl, but not fully. As for the weapon that Luke is using, you have to fight the Sword Master, that giant thing you can first fight when you beat Arietta for the first time. You have to talk to the sword stuck in the floor of the first room in the mansion and beat that demon. The second time I always waited until after Rugnica Plains fell into the Qulipoth and just look around for the search point to fight it a second time. After that, you have to meet Arietta on the Isle of Feres and then watch Eldrant Rise from the ocean. Go back into the mansion and then walk back out and the same sword is stuck in the ground. Fight the thing a third time and you'll get it once you beat him.
It's easier to get the ultimate weapon. All you have to do is complete the single's battle tournament for each chara. That's how they each get their ultimate weapons. Except for Jade who gets it through Din. The ultimate weapon even looks cooler than the sword in the video imo. There are several secrets and such in the game. Many can only be unlocked in the second playthrough while others you have to do throughout the entire game. All Tales games are like that. Even the Titles that give you alternate costumes.
That reminds me. To get some of the coolest lookin' costumes, enter Eldrant once and then leave and talk to everyone you can that hold some power. Talk to Emperor Peony to get some of the coolest costumes in the game and to one of his maids to get costumes that make you think of Power Rangers.