You're really overestimating the Darkrai Buff, I've made a post here before pointing out why Darkrai is balanced for OU and might even help with the meta and should be drop in dlc2.
Psyshock is good but it's not THAT good, the 4 main coverage he will use is still sludge bomb, focus blast, dark pulse and ice beam. Yea you can smack clod and blissey, but so can most special attackers in OU with psyshock, but they don't always use it because of the other better options.
Also considering the power creep and other offensive threats, Its not farfetched to think that darkrai will be balanced.
I've seen that post, and I'm skeptical that Darkrai will use a 4 attacks set as that is wasting the main point of using it, which is to break things. Ice Beam doesn't really do anything major for it aside from OHKOing Gliscor, Landorus-Therian, and Garchomp, which don't even take a boosted Dark Pulse well.
+2 252 SpA Life Orb Darkrai Dark Pulse vs. 244 HP / 252+ SpD Gliscor: 302-356 (85.7 - 101.1%) -- 12.5% chance to OHKO
I remain wholly unconvinced when people talk about powercreep catching up to Darkrai when if released, it'd be the third fastest OU mon without a Choice Scarf or Booster Energy boost. If the opponent sends in a Booster Speed Pokemon, Darkrai can always come in later as the tier is not short of quality U-Turn or Volt Switch users.
The thing is is that Darkrai's closest parallel from previous generations is Naganadel, not Gengar as some people would like to believe, in that they're two powerful and fast Nasty Plot sweepers. Yeah, Darkrai doesn't gain a speed boost every time it gets a KO like Naganadel, but it is even faster and stronger than Naganadel at the onset while having better bulk and a better typing for taking priority moves with it taking Ice Shard, Sucker Punch, and Extreme Speed better than Naganadel does. And unlike Naganadel, Darkrai doesn't get semi-walled by Heatran and AV TTar (Yeah, I know TTar is not relevant this gen, but there is nothing that really beats NP 3 attacks aside from Unaware Clefable, which is not a great set) as it dumpsters both of them with Focus Blast.
Most mons can't afford to run Psyshock, but Dark Pulse and Focus Blast already covers so much that Darkrai can afford to run it while not losing anything aside from the Unaware Clefable matchup. If you predict that Booster Speed Iron Moth will come in, you can use Psyshock, which has a very high chance of OHKOing, which is another plus side of using it over Sludge Bomb. Psyshock also hits Sneasler and Zamazenta harder than anything else Darkrai can run.
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