Interesting Idea here...
Sometimes, when you pick a team member, but can't find particular uses for it that it fulfills by itself, sometimes making it solely a surprise gimmick for your opponent can immediately shut down their attempt to counter you. The idea I had that went along with when using Pokemon like Latias is that Latias is extremely susceptible to Pursuit users, and is unable to beat the majority of them when using obvious sets. To make Latias a killer to these Pokemon, simply adding two optional moves along with Substitute make revenge killing Pursuit users a sort of gimmick by itself. One would think that a Pokemon able to beat it's common counters as its primary role is fascinating, and Latias has that potential.
Here is the set that either OHKO's or 2HKO's Latias's counters Tyranitar, Scizor, and the Ice Shard users Mamoswine and Weavile:
(Not sure about Tyranitar, as it depends on your chosen item as well as Tyranitar's investments in Special Defense and HP)
Latias @ Leftovers/Life Orb - Timid/Modest
4 HP/252 SpA/252 Spe
-Substitute
-HP Fire/Grass Knot
-Grass Knot/Calm Mind/Dragon Pulse
-Calm Mind/Dragon Pulse/Recover
As I said, I'm not sure if Tyranitar is 2HKO'ed by Grass Knot either way with or without Life Orb, if someone can take the time to figure that out for me, that would be nice to confirm otherwise. However, Metagross, Scizor, Magnezone and such are OHKO'ed or 2HKO'ed by HP Fire. Being faster than all of her counters makes Latias ideal for revenge killing those responsible for doing it to her. I've given this thing a try, and swiftly removed the one counter most people produced, Scizor. Grass Knot also lets Latias pwn Swampert out of nowhere. Blissey remains the one reliable switch in, as Latias without Trick cannot do much, and Blissey wins if Latias decides to start a CM war without Refresh, and she loses as well without Recover if Blissey is using Seismic Toss. Because Latias cannot make 101 Subs out of 364 Max HP, she cannot hope to duplicate Jirachi and beat Blissey. However, having 22 more speed points means something! Someone try this set for me!