Ultra League Giratina question

There's an article detailing Giratina-specific counters, weighing the pros and cons of each.
https://pokemongo.gamepress.gg/gearing-ultra-league-giratina

Lapras is a good pick if it has double legacy Ice Beam and Ice Shard, though you may be able to get by with Frost Breath if lacking Ice Shard. Ice Beam is likely a must here due to the lower energy requirement.

The problem with Giratina is that it outlasts pretty much anything. Being one of the earliest adopters of it in my raid group I can attest to there being few things that make me sweat when I bring it out, save for another Giratina. I've used Metagross to handle one before, but you need to be careful in case it's packing Shadow Sneak. I may give Melmetal a go once I get enough candy to evolve one and throw a second attack on it since it lacks the ghost weakness Metagross has.

Steelix is probably the best option. He resist all the likely G moves and deliver SE damage with DT/C.

I use my Aggron!
It's steel, so at worst it'll take neutral damage from Shadow Claw, and at best it'll resist Dragon Breath. It's decently bulky on that front.

It has Dragon Tail fast move which hits Girantina for super effective damage, and you have your choice of Stone edge or Heavy Slam for neutral damage. I find it tends to make the other player panic and use up their shields in response.

Steelix is my other choice. He has Dragon Tail which again does super effective damage, while also resisting his Dragon moves (sporting good bulk too). In addition, he has Crunch which is also super effective and will either make your opponent use up shields fast, or take care of Girantina fairly quickly. (Not as viable in master's league...)

For Master's league, I use my maxed perfect Lugia. He too has dragon tail as a fast move, and sky attack charges up quickly enough to keep Girantina on the run.

The trick for PvP is to resist while delivering some kind of super effective damage to your opponent, even if the pokemon isn't "optimal".