Sarthorael is unstoppable now. It used to be cute that he could avoid magic damage and missile fire when he had a useless lore of magic, so being an anti-archer battering ram was his consolation prize. It's tactically interesting that he basically ignores the mass of enemy units and can go wherever he likes. So he was more monster than wizard and had two unique abilities (70% missile resist and ignore unit mass) as an apology for being forced to use the Lore of Metal. But suddenly, Final Transmutation!
He has magic & missile resistance plus a huge hp pool. The newly-buffed Chaos army is so fast-moving and devastating in melee that some armies need missile fire to to stand a chance, but here comes the giant pin cushion to soak up every random shot and then barge right through the enemy front line to to up their archers. Naturally you can (and should) target your missile units elsewhere and avoid him, but Wood Elves tend to waste a lot of arrows on Sarthorael when you give them move orders and it causes them to switch targets, whereas Empire get their Silver Bullets or whatever tied up in melee by him as the RoR Chaos Spawn, Dragon Ogres and Swords of Chaos charge into flanks and front line unimpeded.
And of course you can't charge him in melee because if you sent one anti-large unit at Sarthorael then he'll respond with dragon ogres or the Swords of Chaos, but if you send 2-3 units to fight them then you get final trans'd! It turns out that the problem with "punishing blobbing" is that it makes monsters units far more powerful, when the army with the monsters has the anti-blobbing spell, and then monsters have anti-large, and they can crash through front lines and engage archers, or buy 400 point warhounds or some flanking Forsaken to do it. This same faction also has some of the best infantry in the game while all this is going on. Chaos don't need to "blob" themselves, it suits them perfectly to have every fight be a 1v1 duel between units, with the occasional monster mash drive-by. So it's a huge Chaos buff. What are you supposed to do?
You can't shot him and you can't chop him, so you ignore him. All of the opponent's anti-monster units are then killed off during the battle, and at the end of the game Sarthorael is strutting around the battlefield like he owns the place having taken barely any damage. Even if the opponent somehow survived the one-two punch of Chaos monsters and infantry, at the end they will have nothing left to kill Sarthorael with. He'll terrorize the remnants off the field or just straight up murder them, himself fighting to the last breath of course as an unbreakable monster.
Of course, not every Chaos player in quick battle knows how to take full advantage of all this, but when they do then it's swift doom for the opposing army.