Short answer: Grind More. Grind until your mouse breaks. Grind until your second mouse breaks. Grind until your finger breaks. Then keep grinding. There is no problem in Ginormo Sword not surmountable by an abundance of grinding. If you think you can't grind anymore, you are probably wrong. The only way to truly grind enough to defeat every challenge Ginormo Sword offers is to start making complicated auto-hotkey scripts to do all of your left-clicking for you while you go to sleep, or go to work, or go out and get drunk so you can forget that you managed to find the motivation to automate your play of Ginormo Sword.
That said, I'm going to assume you mean The Avatar, because he is in fact the actual last boss, and because I can't remember any boss labeled as simply "an incarnation". If you want to beat him, just wear Hyper Armor, pick a Sword, max it out, and enchant it with Darkness to the max. Left click for several minutes, dodging what you can.
Collect gold, repeat.