Long story made short, Rathma damages Meldeln with a black holybolt spray thingy. 5 hits and Mendeln dies.
On the other hand Mendeln shoots white homing holybolts dealing damage over time. Rathma dies from this in around 25 minutes.
Stack up on as much hp and hp regeneration as you can. Mendeln’s AI makes him run away when someone gets to melee range. Use that to bring him to the top corner around the stairs. If Rathma gets closer, make Mendeln move by melee him. Don’t stay too close to him as popping his invulnerability shield stops him from attacking. Rathma takes damage when offscreen. You can stand there and tank him, you just need LOTS of potions. If you see bolts going offscreen to hit Rathma, you are doing it right. In order to safely damage Mendeln, cast an edyrem to trigger his black holybolt spray in Mendeln’s general direction. I like to keep him 1-2 shots from dying, but you can lower them both equally if you feel it better.
At the end, when Rathma haves 1% hp left, make him attack and kill Mendeln. Stay close to Mendeln to get swirlies. It lasts 1 minute. Then take out Rathma via purify, 3-6 shots should be enough. You have 1 minute to take the charm from Rathma and cube it while you still have Mendeln swirlies.
It took me over 30 tries to get this, and it is tedious. But the reward is worth it.