Yaga-Shura has no magic resistance. Fortunately or unfortunately, depending on PoV, that's the only thing he's not highly resistant or immune to.
Anyway, you have Anomen, Anomen can wipe out Shaggy Yaggy with a single Harm spell. Failing that, you'll just have to wolfpack him.
Melissan is invulnerable (yes, she takes damage, no, it doesn't actually matter) as long as any of her named allies is still on the field--your problem with her will be that Yaga-Shura is still there. Balthazar is pretty good for keeping Melissan occupied until you're ready to take her on. Summons don't work very will--it takes her about one round to mince anything that takes physical damage, up to and including a planetar or elemental prince, and Mordenkainen's Swords draw quick Death Spells.
Once Shaggy's down, that just leaves Melissan and her Divine Mantle and Divine Spell Shield. Trent's writeup of the battle (referenced by Wes) includes one bit of misleading information--it states that Divine Mantle cannot be Breached. In fact, it can--it's Divine Spell Shield that absorbs any Breaches cast on Melissan. As far as I can tell, Divine Mantle is nothing more or less than a clerical version of Absolute Immunity, and Divine Spell Shield is a clerical Spell Trap--so hit her with something that will take down a Spell Trap (Pierce Shield, Ruby Ray, and Spellstrike for sure; I'm not sure about what else), followed by Breach. Failing that, +6 weapons, of which there are potentially four in the game, will go through Divine Mantle, as will the claws of CharName's Ravager form. (I'm not sure whether or not the Slayer will work.)
... for some reason, the game designers made all of the uber-bosses immune to Nature's Beauty.
Probably because they read you talking about how great that spell is.
[!--EDIT|Kish|Oct 20 2002, 10:22 PM--]