It's not because Omnigul is hard; it'd because it's long. You have 2 separate parts of which you must "firefight" until they tell you to proceed. Not only that, the final arena in which you fight here is about 1/3 the size of the tower's main floor. WAY TO SMALL!!! You're forced to stay back, either in the entrance or the room at the far right. Otherwise you can sit there and rack up 1,000's of kills. That's how I got my Autorifle & Fusion Rifle Grimoire kills up.