The brave men and women of the Alliance of the Crescent teleport up into near-space, where the Groundpounders can be safely broken.
They cannot be broken in Oerth's atmosphere - the steel pellets flying at those speeds would ignite the atmosphere.
They teleport the dangerous objects away from themselves, and with spells break them, sending the pellets flying ... towards each other.
For the pellets, were they to hit a single Elder One, would simply obliterate that being and continue on, expending very little of the kinetic energy in them.
It is necessary for the pellets to hit each other, for all the energy in them to be released.
For that energy to act on the Elder Ones.
- - -
The pellets streak outward into the oncoming masses of the Elder Ones, in their countless quadrillions.
The pellets strike each other.
Several thousand pellets strike each other, in positions all around Oerth.
Several thousand small Novas appear, the sky over Oerth becoming a hundred times brighter than the sun at high noon.
In a single instant, about 500 quadrillion beings - beings from the depths of the deepest Lovecraftian nightmare - vanish, turned into subatomic particles, filling the skies of Oerth with molten glory.
About two percent of the Elder Ones escape from the destruction visited upon their brethren.
These ones were the ones in the lead, already descending towards the atmosphere of Oerth.
They catch some of the mages and magistresses of the Alliance of the Crescent, but most of these have already teleported to safety.
Then, the Elder Ones meet Welcoming Party Number 2, Oerth-style.
6,000 thermonuclear warheads are Teleported Without Error right into the midst of the largest clumps of surviving Elder Ones.
They may be tough monsters, but hydrogen bombs prove that being ugly and fearsome and strong doesn't mean much in the face of atomic fusion.
Then (see the post below) many thousands more of high powered thermonuclear bombs detonate among them, courtesy of the United Commonwealth of Toril.
Undeterred, the remaining Elder Ones scream for the planet of Oerth.
Even with with under one tenth of one percent of them left, they still represent an awesome force.
But then they meet Welcoming Party Number 3.
The Red Army does not go insane at the sight of Elder Ones - they are already insane, and seeing the Elder Ones just makes them madder.
The Dead that Mina summoned do not go insane at the sight of Elder Ones. Being Dead changes one's mindset and outlook considerably, and the Dead do not fear the living (or semi-living), no matter how gruesome they look.
And the Angels, do not fear the Elder Ones.
The general attitude among all three groups is: This is our war! How dare you crash our party, and try to get involved in our fun? (Well, the Angels don't have that attitude, but they might as well have.)
All three vast armies descend on the hapless Elder Ones, wherever they are, worldwide.
The Angels make sure not a single Elder One is overlooked, and not a single one escapes.
The horror turns into humor, the end-of-the-world scenario ends in farce.
The Elder Ones, expecting an easy victory over the puny, medieval humans, find themselves chopped apart by Angels, Red Gooed by the Red Army (which turns them into Red Goo Monsters, and forces the Elder Ones to serve humans!!! That has got to be a first!), and as for the Dead ... they put their hands right THROUGH the flesh of the Elder Ones, and proceed to pull assorted internal organs right out of their bodies - internal organs even the Elder Ones require, such as their brains, hearts, and the like.
Those Elder Ones that somehow manage to break through the Angels, the Dead, and the Red Army, meet the armies of Oerth, led by Hazen, Iuz, Bigby, the God Emperor, Kalanyr, Vaeregoth, Kas, Shyntara, and all the rest of Oerth's great heroes and villains.
Complete with special magical protections against insanity.
And with high powered, magical weapons ... you know, like magical rockets, magical machine guns, magical artillery and mortars, magical tanks, and when all else fails, some really nasty magical swords.
The hapless Elder Ones are slaughtered.
Those that try to run from one force, run right into another force.
Those that try to run downward into the planet, run into the formians.
Those that try to run back upward, run into the Angels.
Those that try to go to other Planes, run into a very large, very angry githyanki army that was put there as Welcoming Party Number 4.
In the end, not a single one escapes.
The Elder Ones, are destroyed, down to the least, and down to the very last.
- - -
For Lord Melkor, it is back to the old drawing board.
By the way, he is only dead for one day In Character.
Suicide doesn't kill him for very long - to kill Lord Melkor for any important length of time is going to require that someone else kill him beside himself.