Latias can do a lot of things. Its movepool is filled with both supportive and offensive options, receiving moves like Tailwind, Ally Switch, Helping Hand, Psych Up, Recover, Healing Wish, Charm, Psycho Shift, Wish, Heal Pulse, Icy Wind, Calm Mind, and so on. It just lacks Trick Room and Skill Swap. And although its signature attack Mist Ball is pretty bad (why don't you just trigger 100% of the time like Mystical Fire?), it doesn't mind too much as it has plenty of other options.
Latias @ Weakness Policy
Ability: Levitate
252 Def, 252 Speed, 4 HP
Timid nature
- Calm Mind
- Stored Power
- Shadow Ball
- Protect / Dragon Pulse
One of the many ways Latias can run in doubles. Ideally you pair this up with a fast pokémon that can Fling a Starf Berry to Latias. Because it's a berry it won't really deal any noteworthy damage, but it will trigger Weakness Policy. If you also set up a single Calm Mind, essentially you reach +3 Sp.Atk, +1 Sp.Def, +2 Attack, and random +2 in one turn, to a total of eight stages of stat boosts. After this, Stored Power becomes a massive weapon of destruction with 180 base power at +3 Sp.Atk (or +5 if Starf Berry boosted Sp.Atk), even when uninvested. Max speed to be as fast as possible for attacking, max defence to take physical hits better, relying on Calm Mind and a naturally high Sp.Def stat to keep the special side safe. Shadow Ball is there to hit Mega Metagross, Aegislash, and (in the case of VGC19) Dusk Mane Necrozma and Solgaleo, as well as having an attack that's neutral on most other steel types. The last move can be Dragon Pulse for additional STAB, Protect because it is doubles, or you can even fit in different things like Recover or Ally Switch here.
Even without Starf Berry support it's still a solid option. Latias has quite a number of weaknesses, so triggering the Weakness Policy isn't all that rare (and with Latias's solid bulk it can generally survive super effective hits too), and if you get a Calm Mind + Weakness Policy you're already at +3 Sp.Atk and Stored Power is going to hit like a nuke with 140 base power.
Another supportive option you can do with this is Simple Beam support. If you Simple Beam Latias right before it sets up all those stat boosts, including the Weakness Policy, are doubled. The downside is that Simple Beam is relatively scarcely distributed and no really fast pokémon get it, the fastest is Linoone at base speed 100. Xatu could be an option there, it is immune to Taunt and Encore, has decent speed, and can also set up Tailwind, although its bulk is somewhat lacking.