Lugia, the great wall of Pokémon, the golden standard that all defensive walls are compared to. Sadly several generations of power Creep have caught up to Lugia, preventing it from walling as effectively as it used. Lugia's typing, while not bad, isn't good either, with numerous weaknesses that other Pokémon can prey upon and lacking a wide myriad of resistances that other defensive staples use to excel. Lugia's reliance on Multiscale, is a big liability, meaning any passive damage (Sandstorm, Stealth Rock, etc.) heavily impacts its defensive capabilities.
Lugia's passivity makes it regulated to stall -thankfully it isn't easy setup fodder thanks to Whirlwind, but will often be forced to Roost/Recover and rely on Toxic for damage.
However, some of the generational changes do work in Lugia's favor. Lugia can actually make for a terrifying Dynamax sweeper. STAB Max Airstream is never something to scoff at, and its titanic bulk before Dynamaxing makes triggering a Weakness Policy boost rather easy, allowing Lugia to make up for its below average Base offenses. Lugia won't run over teams immediately, but its titanic bulk can make it an unstoppable force few can reckon with.
Ruler Beneath the Sea
Lugia
Ability: Multiscale
Item: Weakness Policy
-Aeroblast
-Roost
-Calm Mind
-Earth Power
Nature: Bold (+DEF, -ATK)
EVs: 252 HP / 100 DEF / 156 SPE
Lugia can take advantage of the switches it causes to setup a Calm Mind, grab a Weakness Policy boost when your opponent inevitably target's Lugia's weakness to take it down quickly, Dynamax, escalate out of control with Max Airstream and go for the sweep. Investment is mostly into HP and Defense to maximize bulk. 156 speed is necessary to outspeed Timid Shadow Rider Calyrex at +1. You will be leveraging Calm Mind and Max Quake (which has extremely complimentary coverage with Max Airstream) to bolster Lugia's Special Defense.
Checks and Counters
Any pokémon that can outright ignore Multiscale immediately has Lugia on the backfoot. Zekrom and Lunala with Terravolt and Moongeist Beam can outright ignore Multiscale and hit hard with Super Effective STAB. Necrozma-DM lacks the same immediate power but can still do big damage with Sunsteel Strike. Calyrex-Ice and Shadow Riders both easily 2HKO with their STABs, regardless of Multiscale, though Roost can mess with Ice Rider. Tyranitar also excels, its typing giving two dangerous STABs to Lugia and Sand Stream rendering Multiscale useless.