About Isuka, after level 100, you actually go in increments of 50.
It doesn't help that along the way, the opponents will have gold properties without the damage. Johnny will teleport when he dashes, but his Mist Finer won't instakill you (thank god.)
It's really just a matter of patience, since you eventually get life regen and vampire heal attacks after a certain level.
Then again, I did beat level 999 on Beginner.