Shout out to u/straightuprat for suggesting me an idea, that I latered turned into this strat.
Basically what you have to do is get an Uber Klaus Core to start the self destruct sequence and then brainrot him so he blows up a normal Uber Klaus, since friendly damage without brainrot is no longer a thing.
In order to spawn 2 Uber Klaus on directed all you need to do is progress until you reach the step where you need to disable the shield on an Uber Klaus in front of the forsaken, this step triggers one to spawn each round. But since the core is a Special enemy now and no longer Elite, you can't just leave it alive to progress a round, instead what you need to do is to weaken the full one (I suggest away from the Forsaken to not accidentally start the boss fight) then end the round and quickly kill him before the next one starts, this will allow you to start a fresh round with a Core and a full one will also spawn.
Now you need to kill every zombie and only leave these 2 alive (since Cores are special you won't progress the rounds) buy a weak preferibly green gun and give it brainrot, and then make sure to weaken both with your main gun without killing the normal one or triggering the self destruct, once you're ready and at a safe distance trigger the self destruct, quickly switch to your brainrot gun and pray you turn him before be dies, since there's no more zombies he'll attack the Uber Klaus and that's it.
Directed mode makes it a lot easier because you're stuck on round 10, and if you mess up you can always just save and load and stay on the same round. I recommend you to not hurry and take your time buying perks and armor until you feel comfortable but make sure to not upgrade your main gun too much or it'll be hard to properly weaken them.
Hope you find this useful, I had lost hope after the patch that disabled friendly damage, but now it's arguably even easier than before using Directed.