You might want to just start a new game with the same character. You should be able to rush through the dungeon to a a few levels before where you are stuck and get a few extra level ups. You keep all your spells, gear, inventory, and char level so no real drawback.
And all this makes me wanna play it again…
So I don’t remember specifics of the skills or gear I used, but I do remember it being crucial to take advantage of the layout of the levels - doors and choke points.
I assume you did all the side quests?
Edit: that video you posted- I don’t remember it being so hard as it looks in the video. I know I spent a lot of money gambling for better gear
Try Deus Ex