If I activate {Jace's Archivist} activated ability, would my {Psychosis Crawler} die?
I know it's a "discard, then draw" but since it's one ability, Does it happen simultaneously?
It wouldn't. Yes your crawler would be a 0/0 during the resolution of the ability however state-based is only checked before an after the ability not during so it wont die.
As a general rule just remember you can do anything in the middle of doing something else.
RESOLVED
Thanks. I'm was pretty sure that's what it was but wanted to make sure bc I love that combo.