Not kidding. It lasts only (seriously) not even two seconds. And in the manga, it's just 2 panels. But it's still one of my favorites.
Hawkeye vs Envy from Brotherhood episode 54.
Envy transforms his/her/it's arm into a sort of green (Oh-hoh. Green with Envy. I see what you did there Arakawa) spear of sharpness. Aims for Riza's face, Hawkeye narrowly dodges, and uses the momentum from the dodge to whip her rifle around, and blow out Envy's knee cap. This happens in the span of 2 seconds. No joke. I love it.
(I included two other shots, because they are awesome)
"Thanks for falling for it, now do me another favor and die"
Thank you, Riza Hawkeye. For being an amazingly awesome character.