Or you could possibly have to pass an extremely high repair and science check (combined) in order to activate the robot for use, if not, then you must battle through the Enclave troops leading to the Jefferson Memorial yourself with the Lyons Pride. Conceptually, it could be altered so that the issue with Liberty Prime itself is not improper functioning, but rather that the systems aren't routed properly, thus making the player's ability to repair it plausible.
If the player does not repair Liberty Prime, he could possibly destroy the barriers blocking his path with high Explosives, thus allowing the Brotherhood to advance quickly but with increased resistance as the Enclave recognizes the use of powerful weaponry. A Science player who does not have adequate repair skill to activate Liberty Prime could hack the grid powering the force fields and shut them all down, providing quick advancement, on the other side of the coin, a player with high Repair could fiddle with the construct of individual forcefields and shut them down one by one, not as effective as a Science player's result, but still an option.
To make it more difficult, there could be far more Enclave troops occupying the path to the Memorial, there could also be Speech checks with various random Brotherhood members to provide fodder for the attack, like the Enclave troops at the end of Fallout 2.
Rivet City might also be convinced to lend some of their security teams, since the city itself is crippled by the irradiated waters surrounding it, the player could inform them of the specifics of the program and convince them to provide some support, there could be Intelligence checks to convince them that if the Enclave captures the Purifier, there would be no hope of free water without consequence, but if the Brotherhood takes hold of it, their problems with the sickly waters surrounding their home would be solved without much of a price beyond a fierce battle.
Charisma or Speech checks might rally them behind you with pure vehemence against the Enclave, but Intelligence would provide substantial reasoning that will provide many more security members from Rivet City rallying behind you as you have given them a clear goal and cause.
There could be several other ideas implemented. Know how the Enclave utilizes mortars against the Brotherhood during the battle? A Big Guns or Explosives character could possibly help construct or rig similar weaponry for the Brotherhood and thus soften up the Enclave forces with artillery bombardment.
There are countless other possibilities as well, medical help boosting the abilities of your fellow Brotherhood members in battle with a high Medicine skill perhaps? Tons of plausible and sensible things to consider in improving the endgame.