Giratina Counters

I don't know for sure as I didn't run a calculator or do PvP myself.

However I checked the charge moves on this site, and dragon claw and Shadow sneak do the same damage, but dragon claw costs 10 less energy.
I also believe the typical fast move for giratina is dragon breath.

So while steel types are great at reducing that dragon damage and forcing giratina to use the less frequently firing Shadow sneak - Snorlax is getting hit for neutral+stab by those dragon moves. While the SE multiplier is larger than STAB, giratina will be better at blowing through shields with a better moveset.

I'm uncertain that Snorlax would be a clear winner in the matchup. I'd guess he wins with no shields, but if each uses 1 or 2 shields I don't know (for two, I'm tempted to guess giratina wins).

If this is the case and your party includes Snorlax as your only giratina counter, then they could safely lead off with giratina knowing in a full shields matchup they win. However you may be able to generate a shield advantage using this matchup at least.

Snorlax wins over Giratina only if he is able to bait shields with BS.

I have used both maxed Steelix (DT/C+EQ) and Snorlax (Luck/BS+OR) as attackers against Girantina (which I also own with two charge moves), and feel as though Steelix lasts longer in the long run.
His resistence is great, and crunch is super effective against Girantina.

I've also used both options and strongly prefer Steelix over Snorlax. The much higher defense stat and resistance to both dragon and rock gives Steelix significantly better staying power compared to Snorlax's larger health pool and immunity-tier resistance to ghost (which most Giratina won't be using anyways). Plus, Steelix is able to fire off Crunch at a much faster rate than Snorlax can fire off Outrage due to it costing 15 less energy and Dragon Tail having slightly better EPT than Lick.

Snarl/IB suicune's great agaisnt giratina too.

But as for the snorlax or steelix, it comes down to what you have, if you got a high level boosted onix, i'd go for evolving that over powering a snorlax because it could cost less. if you dont have a high level onix, i'd say go for snorlax, but eventually you will want to get steelix, it is much better.

I have 3 Giratina counters that I like. They all do well. IS/IB Lapras, DT/C/E Steelix, and DT/SE/HS Aggron. Really can't go wrong with any, but Steelix is probably the best. My favorite is the Lapras, but most to not have access to Legacy Lapras.