He basically used an item called a Granade, which early on had a chance to permanently increase your stats. By combining this with Save-Scumming, he used them to… spam “BUFFS” which gave him arbitrarily high stats in everything. Since it’s a percentage based increase on the -current- level of stats, it gets increasingly ridiculous the more Granade’s you find.
It’s no longer possible to do it to -quite- that same extent, but there are other ways still of breaking the shit out of your character. My current method is to go Cephalopod post-threshhold, which DOUBLES your intelligence, and doesn’t check if intelligence is temporary or not. So you can use it to obtain an intelligence of around 80 or so if you stack every imaginable buff together. Then you take the Niten Ichi Ren fighting style, which takes your Intelligence, and uses it as a modifier on your To-Hit and Crit ratings, so you can crit hulks to death in seconds. I haven’t done this in awhile since Glass-cannoning isn’t really my typical style…
The other method is: Cyborgifying, and Laser Weapons. An extremely old cyborg will accumulate extreme amounts of Bionic Power, and can combine this with a heavily modded A7 Laser Rifle to make the whole game quite trivial. You can shoot a whole horde of zombies to death, and then eat their clothing to regain energy to keep killing, and solo entire cities.