Just unlocked it... and again, thanks to Xbox One Game DVR I can show you how i got it.
After multiple attempts, i think i figured it out.
#1.You have to do this in a match.
#2. I believe that the "levolution" event in Caspian Border cannot have been triggered, meaning, the tower needs to be still standing.
#3. The gate at the end/beginning of the tunnel (depending on which way you're entering it from) needs to be broken by YOU
Good luck, and get to that tunnel/gate fast!