If it's really that steep
Even the best riders in the world struggle.
Let's assume, for the moment, you have a compact crank and a 28 tooth on the back.
A cadence of 60 rpm with that gearing - that's very slow pedaling, and it *hurts* to maintain high wattages pedaling that slowly - moves you at about 9 km/h.
To maintain 9 km/h on a 25% slope would require a power output of around 470 watts, assuming a 68 kg rider on a 7 kg bike. That's a required power output of 6.8 W/Kg.
Even Contador can't manage that for more than a few minutes.
Aside from losing all the weight you possibly can, one other thing that I've found helps with really steep climbing is upper body strength, so you can stabilize the bike with your upper body while forcing the pedal down. Oh, and if there's less-steep climbing before you hit this monster wall, pace yourself so that you can put in maximum effort up the difficult part of the climb.
That said, there are very few sealed roads in the world that maintain those kinds of gradients for more than a couple of hundred meters.