activated its Quick Claw!
You're right. What about going mixed with Bullet Seed? It still isn't going to kill Latias, but Bullet Seed has a massive 48 PP, which should be enough to outlast Recover PP. And it has enough power to kill Quagsire, even without Attack investment:The main issue with using Octillery is that no matter which coverage move you choose, there will likely be some stat booster that you can't break without a critical hit, like the Calm Mind Latis with Energy Ball or Calm Mind Suicune with Ice Beam.
0 Atk Octillery Bullet Seed (3 hits) vs. 252 HP / 0 Def Unaware Quagsire: 144-180 (71.2 - 89.1%) -- guaranteed 2HKO after Leftovers recovery
Bullet Seed also allows you to pray for a small crit if you need to, since with 48 PP and 2-5 crit chances per attack there definitely will be a few of them along the way.