See, the problem is that the built-in combat system just plain doesn't work. And it was never meant to. It was a copy of the man-to-man system in Chainmail, polished and tweaked, and that system was meant for foes of roughly equal size and capability to fight each other. It's legacy code, if you will, and it's currently being used for things it was never meant to do.
So what would I do?
I'd use a completely different system. Because you're not "in combat" with giant godlings as much as you endure and trick them. You "fight" them as much as you "fight" a trap in a dungeon. You either outsmart it and live, or you fall prey to it and die.
If you've ever played God of War, you know what I'm talking about. Kratos doesn't swing his chain sword things at Ares or Gaia or whatever, he has himself a Quick-Time Event and climbs their faces and stabs them in the eyeball. In Dragon's Dogma, if you want to beat the Griffon, it helps if you cling to it and try to pin its wings down so it can't fly away (although that never worked for me).
So why can't you do that in a tabletop game?
So here's this guy. He's like a hundred feet tall, or something. He has trees growing into his back. His stat block is like "HP: 2,000 AC:22, #AT:2 DMG: 6d6+30". If you get swatted by that bigass hammer, you are going to be a dead man, and if you stand there and beat him up, well, see you in the afterlife.
So what do you do?
You make it part of the world. You make him vulnerable to a certain sword, or artifact, or spear. L'lurgohd up there, turns out he's vulnerable to a spear made of solidified light, dipped in the blood of a demigod's heart. So the adventurers go off and have themselves a badass quest where, after a difficult and painful struggle, manage to attain the Spear of Vengeance, and give it to the burliest fighter. Here's their plan: They need to distract the beast so they can plunge it into the roof of the beast's mouth, peircing its brain.
If you're using a system with lots of skill systems, you could make it pretty easy: Let them use their appropriate skills to study ways to bind, trap, or confuse the monster so that the burly warrior can get in there. Maybe their plan is to lure the beast towards a ballista attached to a rope, so they can pull the thing down before it notices, and then once it's down, they stun it with the potent Ritual of Elnor'Yorgo, a spell the party cleric and wizard had been working on for two months. The ballista works, but the spell fails (bad spellcasting check), so the beast gets up. What will they do?
They get desperate, and the fighter leaps on to the monster's chest, while the party desperately distracts it by stabbing at its toes, casting spells on its eyeballs or its hands, creating illusions, anything they can do to make it not notice the tiny man climbing its body.
In game terms, what you're doing is a lot like skill challenges in 4th edition, in a way nobody thought of it. You let the players use all their skills to make things happen, and they're going to need to make more checks than they fail. Their Dungeoneering skill might not be useful when fighting L'lurgohd, but their Religion skill is (recalling the ritual). A good check gives a bonus on the spellcasting attempt (Arcana, and the cleric is Aiding Another while the wizard casts), and if that succeeds, it gives a massive bonus to the fighter's Athletics check to climb the beast.
But you throw in a wrinkle- a skill challenge within a skill challenge. The beast noticed, and so everybody's doing what they can to help out the fighter. Mechanically, it's all the same as an Assist Another check, but give bonuses to the roll if it is more helpful. Rogue aiming at its eyeballs? Paladin going back to the ballista for another shot? Wizard summoning tentacles from hell to grab its toes?
And here's the fun part- once you realize the possibilities, you can do a lot of things with it. You can bring in the "knock them down and then bash them in the head" by just having the players make skill checks. Now it's prone and its relatively unarmored head is only 15 AC, and also it takes double damage because that's it's weak spot.
I mean, honestly, I'm surprised this isn't a thing yet. There's got to be somebody out there who's done a better job at exactly this sort of thing than me.