Lions had a game plan of keeping a high line and compact midfield to unsettle Broadfields rhythm and it worked, restricting the home side throughout.
Adam Bolle and Adam Arnold went close in the first half and the beck four held firm when United showed their danger from wide areas.
After the interval it was Lions who looked the more likely, and missed a great opportunity to take the lead when Liron Mannie was tripped in the box, only for the usually reliable Adam Bolle to drag the ball wide from the spot.
With the game level at 90 minutes, the tie went straight to penalties. Both sides missed their opening kick, before David Myers denied United a second time. Arnold, Ollie Craig, Myeroff, Zac Rose, Reiss Mogilner and Adam Harvey all netted to send Lions home 6-5 winners.
Lions go into the final having made at least one major final for the fourth season running.
Andy Landesberg's men were 1-0 winners over Buckingham Athletic in their most recent league game, with Ollie Craig the match-winner.
