No, no one beat the Iron Siege...
Really? Invent your own supermegawidened Dimensional Lock or Forbiddence effect. Grab a Reverine Ethereal Reaver, shift over to the Ethereal Plane, and simply walk in and start whaling on him. None of his abilities cross planar boundaries nor does he have any ability to Planes Shift over unless he turns tail and runs away advocating the throne to you like a cowardly outsider that'll never command the fear and respect of his follow demon/devils again. So his one, and only one, option is to sunder the blade of your sword. Except it's immune to damage so he has to successfully Distinction it first and the Will Save on that is only DC 38 or so. Heck you would even use his highest Save DCs, 64. If you assign 16 Divine Ranks of the eighteen you were given to the same character you get a +75 bonus to rolls for ability score generation automatically taking you to the ability cap of +30, and at 70HD even a poor Will Save comes out to +23, throw on a Vest of Resistance +10 rather than a much larger Epic Spell and you still
only fail on a Natural 1 and have over thirty one Feats to pick up a Luck Reroll or two to negate that. There ya go, immunity to all but his Alter Reality access to Force Spells which caps him at using something like Wings of Flurry for CLd6/rnd. And the only reason he has that isn't some author foresight to counter basic tactics but all deities inherently have it as part of their lazy writing, including your characters.
And if you want to get even simpler, Monk 7 / Lion Totem Barbarian 1 / Outsider 62 with 16 Divine Ranks gets +20 Immediate Actions and can make a Skill Checks using them. You run in to attack him, he uses his 1st Immediate in response to make an Iron Curtain or something equally ignorable such as any 9th level Quickened Spell which he can't do anyway, so you use your 1st Immediate Action in response to that to Jump at him to trigger Mantis Leap to trigger a Pouncing Charge. Now he has to use his next Immediate Action to block the same exact tactic yet again and spoiler alert, he runs out first. So you charge him and he dies, his True Resurrection resolves, and you finish your first full-attack and he dies again. Because who can't deal three thousand damage per round using an uber charger before level 20? Edit - Just realized he has no noted Foresight or immunity to being Flat-Footed, you can negate all of his Immediate Actions by using Shadow Blend or even Darkstalker and simply beating his Spot Check.
There must be some kind of massive thing that's not mentioned in the thread. Like the Iron Tower has six hundred and sixty six clones in it that all have to be killed within six minutes or something. Oh, and it's a six hundred thousand mile wide fortress that it's entirely constructed out of moving Void Stone traps and twisting interlocking Mystic Admixtured Forbiddenance effects so you take like 18d6 damage for every 10ft you move and risk being wiped out of existence for a wrong step as punishment for being to hasty.