Gutting Gastrodon
Gastrodon is an amazing bulky water pokémon that shuts down a myriad of threats. Most water types, and any electric type lacking HP Grass/Energy Ball find Gastrodon to be an insurmountable wall. Everything else though... can give Gastrodon serious problems.
Obviously, anything with a grass move has a quick ticket to OHKO the sea slug, as the quad weakness is too crippling. This + Grassnium Z means that Gastrodon can no longer comfortably wall Fire types the way it used to. Most fully evolved Fire types have Solar Beam, so that plus Grassnium Z means a surprise Bloom Doom for Gastrodon, though most fire types cannot come in safely. As expected many Grass types are Gastrodon's kryptonite, resisting both of its stabs and able to OHKO it with ease, though the ability to survive an Ice Beam on the switch in is a must to be a true counter (i.e. Mega Venasaur, Ferrothorn, Tangrowth, etc.). Additionally, Gastrodon's lackluster Physical defense leaves it vulnerable to most physical sweepers: Salamence, Dragonite, Azumarill, Garchomp, Medicham, Absol, Conkeldurr, etc. All of these can either punch through Gastrodon, or boost up and muscle through its defenses. Gastrodon's special defense, while significantly better, does not exactly make Gastrodon proof against the stronger special attackers like Latias, Hydreigon, Chandelure, etc. who can quickly 1-3HKO the offending slug. Gastrodon's slow speed does it no favors outside of trick room, as it must take a hit before doing anything.
As with any tank, Toxic and Taunt. Taunt is down right crippling, preventing Gastrodon from recovering, buffing and/or debuffing. Toxic is horrific to Gastrodon as well, ruining its tanking ability lest it carries Rest (and since it has access to recover, that is almost certainly no).
As most Gastrodons carry Storm Drain, their own ability can be used against them. (Mega) Alakazam, Gardevoir and Porygon 2 can Trace Storm Drain to buff their SATK, and quickly OHKO with a boosted STAB move/Energy Ball (or stall it out in the case of Porygon 2). Skill Swap, though niche, can be used to similar effect.