Offensive Answers
252+ SpA Choice Specs Tapu Lele Psychic vs. 252 HP / 4 SpD Toxapex in Psychic Terrain: 462-546 (151.9 - 179.6%) -- guaranteed OHKO
252+ SpA Choice Specs Tapu Lele Psychic vs. 248 HP / 0 SpD Zapdos in Psychic Terrain: 343-405 (89.5 - 105.7%) -- 37.5% chance to OHKO
252+ SpA Choice Specs Tapu Lele Psychic vs. 252 HP / 252+ SpD Landorus-Therian in Psychic Terrain: 262-309 (68.5 - 80.8%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Choice Specs Tapu Lele Psychic vs. 252 HP / 248+ SpD Hippowdon in Psychic Terrain: 280-330 (66.6 - 78.5%) -- guaranteed 2HKO after Leftovers recovery
Both Timid and Modest work, also Specs Lele can absolutely be ran with both Psychic and Psyshock, if you absolutely want to run tbolt, just replace moonblast.
The only mon that somewhat checks Blace on the core is Pex, however after a few interactions it is fairly simple to break down, with either a spdef drop occuring or some other kind of chip (Usually rocks), making pex unable to be a long termcounter.
The grounds are barely checks to SD Chomp, if you're a real one you run Aqua Tail though
Water and Electric coverage hits both Pex and your bulky ground, while zap usually has no way to realistically threaten rotom.
Galarian slowbro has an advantageous matchup into the core, by threatening pex and zapdos hard, while being able to threaten a ground type switchin with either scald or a sludge bomb poison. Or if you're a real one, you run Shuca NP with Ice Beam/Psychic/Flame.
Less Offensive Answers
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I hate this guy as much as anyone else, but it lives forever and generally annoys all of the mons listed with knock and can either set up rocks or somewhat freely moonblast.
Future Sight
While not an instantaneous means to threaten a pokemon out, future sight + a breaker has proven to be a very effective way to beat cores with pex.