slay that unicorn
Overcoming an obstacle that has eluded for so long as to seem impossible.
In trick dog training, a Unicorn trick is the trick that has been so difficult to achieve that it seems like a mythical construct - an impossible dream. Thus, Slay that Unicorn is finally accomplishing the impossible!
slay a unicorn
verb- The act of someone hooking up with one of their dream girls.
Holy shit dude I just got to slay a unicorn, I'm just calling to tell someone so I know it really happened later!