Doom and Night work about half the time (give or take). Doom still gives them time to take a turn or two, not to mention a counter. Night, if successful, puts them to sleep, but they are so quick that if you aren't casting night with every one of Quina's turns, they are roused and rude in no time.

In my experience, Night is worth the trouble, though, and so is doom. Skills/ summons/ etc. that hit more than one Yan are very risky, because if more than one of them counter with snort, you're liable to be left with no one on the battlefield.

My advice is this: be able (that is- more likely) to survive comet, and save after every one or two battles. And if you're not doing max damage with each attack, you're wasting your time.