This boss fight is pretty simple, but not in hardcore draconian mode with all settings on. Then this fight is a nightmare. I spent hours and hours on this fight, and looked up multiple guides and absolutely nothing worked for me. Between no armor and shypox this fight is absolutely the worst so far for me. I wanted to make this because I believe I understand the fight very well now and have a strategy to help others in the future.

the best greatsword you can get, I had a demonsbane +3 which was the best statted weapon I could get my hands on that early

you absolutely need to be high enough level to have learned the moreheal spell, but I wouldnt recommend going any higher than 40 as the higher level you are past that the harder this fight actually gets

you need a move to target a group of enemies, helichopter is what I used but it could at times be inconsistent, however rare

Right away Rab blasts you with his power move, and you have to heal. This move always does a fixed amount of your base hp, nothing you can do about it. This means the higher level you are, the more damage it does, and the less likely you are able to heal all of the damage. But its Draconian, and if you are like me, up until this point you've been grinding levels a bit and are pretty up there, I was 42 when I finally beat him, and I regretted it. Shypox is also a big factor, keep in mind no matter what, this fight is RNG based because of shypox, and you will have to give it a good amount of tries no matter what. Rab has 3 moves: he can attack you, he can summon clones, and he can charge up his power move, and the turn after hit you. (I've seen him charge and unleash in the same turn though, not counting the very first turn) His clones can attack, cast crackle or zam, or just do nothing.
The clones can run out of mana, and most guides I read labeled this as the best strategy to take, but this absolutely never worked for me, not even close. there is also one mechanic I noticed through my many runs of the fight. Rab has 2 checks during the fight, and if they are met he stops whatever he is doing, even if he was charging his power move, and on the next turn will try to summon clones. This was a constant for me every single time. the very first time you quadraslash him, and the fourth time you hit him. NOT the first and fourth time you use quadraslash, but the first and 4th time you hit him with it. This is very important.

Here goes. First off you need a way to kill the clones, both of them, in one turn. For me, helichopter worked, but gigaslash and gigamash should absolutely work. helichopter only works if you have a high level (keep in mind I was 42) and a maxed out demonsbane, and even then its not 100% I noticed. The fight will always start with you getting hit, so you need to heal, then he hits you twice more and you need to heal again. On the turn that Rab attacks, and then charges his powermove, you then quadraslash. Instead of attacking, he will now summon clones, which you need to remove immediately. If shypox happens on the same turn as a clone spawn its looking grim, and you may end up spending a lot of mana trying to heal back to stabilization. In general, assuming you have the hp to do so, the next 2 times you use quadraslash on Rab you will need to do so at some point on a turn after killing the clones. This means you will always get hit by Rab's power move, but you will then get chances to heal. The fourth time you use quadraslash is important, instead of using it on Rab, which will reset his next move to clones again, you want to use this one on a clone. It should do 150 damage and kill the clone. If you survive and have even a little health left at this point, you're golden!
Next turn no matter what, quadraslash Rab and it will do 300 damage, AND it will reset his next move to summon more clones. This means your next move you are free to quadraslash again, which will win you the fight! I don't know Rab's exact HP, but skipping the fourth quadraslash on him does not matter as long as you hit the fifth and the sixth. This is ideal because hitting the fourth on him, resetting him, and then hitting the fifth, DOES NOT KILL HIM. You might ask why use the quadraslash on a clone and not just on him, well if you do that then you reset him on the fourth and get the free hit for the fifth, which wont kill him. but skipping the fourth on him and going for fifth into sixth, you can abuse the rest to finish him off. Obviously due to shypox you'll have to try a couple times to achieve all this. I really hope this helps someone since the fight for me was so hard, and no guides I found worked for me. In the fight I won, I was affected by shypox 3 times. I was absolutely blown away that you didnt need a perfect run, or even a near perfect run. Admittedly I was lucky with the attacks that the clones used at the end though.

Other guides I read basically had 2 strategies. Either you leave the clones alive forever until they run out of mana and then play the fight out until you win, or a strategy with miracle slash that kept one clone alive at all times that supposedly kept Rab from summoning more clones. I did not try that second one, but the first one was absolutely a no go for me. Shypox was one thing, but leaving the clones alive did way to much damage to me, and I actually couldn't get them to run out of mana before I died. Not once. Regardless, if my strategy isn't working out for you these are some things that might. I highly recommend if you try another strategy to still make use of the first and fourth hit reset on Rab that I mentioned earlier, as I've not seen other guides mention this.

I hope this helps someone, even just one person. The amount of time I spent on this fight was pretty frustrating, and I hope no one has to go through it again.