Drones and Screamers are going to have excellent cover and invuln saves, and are excellent at killing big MCs or vehicles, respectively. They aren't distraction units so much as really tough, quite mobile bubble-wrap for his Heralds. His list is great at scoring objectives--summoning units if/where it needs to, doing a bunch of psychic and save-boosting shenanigans, etc.
It doesn't have too much in the way of anti-horde, but don't assume that means an easy win--this list looks basically like a netlist copy/paste of Nick Nanavati's list he's used to win several of the biggest tourneys in the States (BAO and Adepticon, IIRC).
Singling out the Heralds would be a key move--especially if you can kill the Herald of Tzeentch that has the Exalted Reward (Grimoire). Secondary is the FNP-granting Herald of Nurgle, but also any Herald that rolled a Cursed Earth or the Telepathy power that grants a 4+ invuln. Fateweaver would be great to kill, since he's the very expensive lynchpin of the list's synergy, but the best way to take him down might be by Dakkajet, honestly (and if he rolls Invisibility and casts on self, or uses Grimoire on Fateweaver to give him a rerollable 2+ invuln save... well, ignore Fateweaver and be happy he didn't target the Screamerstar or Dronestar with that power or buff).
The Drones will always have a 2+ cover save, and are many-wound T5 minis, so power klaws might be the way to take them down. They don't have much in the way of AP2, apart from the 2 Heralds of Nurgle that will be tagging along with them on foot, to boot, but do beware their one Instant Death attack per assault phase stings.
One squad of Screamers... well, are likely to be Grimoired and buffed in some other way, so (as long as the Grimoire Herald is alive) they have an 8/9ths chance every turn to have a 3+ or 2+ invuln save, always rerolling 1s. Plus they have an armorbane attack, so Battlewagons are probably not the best choice--but with only those 2 Screamer units as his main anti-tank (and some witchfires from Fateweaver), massed trukks might do pretty well letting you close the distance against his mobile units... and providing you bubble wrap he has to crack with his minimal shooting or wasted assaults.
It's a deceptively tough, mobile, and flexible list. Pray that your opponent is netlisting flavor of the month and doesn't know how to play it well, and do your best to snipe his important synergy characters.
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!