Hold onto it, unless you really plan to take down Airus and go for the dark prizes. And even then you'd need more slayer cards I believe. I also saw your rank, and it seems you're doing fine there. So yeah, I'd keep it. Maybe next elite guard or next event overall will have a useful card that'll stay useful the whole event.
Yeah, all hope is lost. I would only use it to get one of the cards. Any cards for more SED, not for a slayer since I've pretty much given up on Airus.