Liberty Bowl is relatively steep -- sections in the 40 degree range and most of it in the mid-30's. It is definitely off-piste skiing.
If you want to ski Liberty Bowl, then you need to seriously step up your skiing. I'd recommend taking some lessons. There's plenty of steep off-piste terrain to learn on at Big Sky. If you take lessons, they will work you over teaching you the skills necessary to ski steep off-piste terrain. That includes learning how to self-arrest, as a slide down a 30+ degree 1000+ foot bowl not only risks bodily harm, but possibly death (although Liberty Bowl has few of the terrain features that pose larger risks to life and limb, like trees and rock bands).
As others have suggested, you can start on Andesite Mountain. If you can handle those slopes, then you might try the triple chair into the bowl, where you can traverse into steeper lines that are relatively short. These will have moguls, but they are small moguls (usually). If you can handle that, then it's time to test yourself on the Challenger chair, where you should first attempt Moonlight (the run right next to Moonlight Basin). It will have, most likely, large moguls and has an initial and ending section that are relatively steep. If you can handle that run, then some of the other lines off of Challenger might be attempted next, such as Big Rock Tongue (traversing into from the left side -- it is very steep if you come in from the top). If you can handle that, then you are ready to attempt Liberty Bowl.
Off of the top of Lone Peak are some of my favorite runs anywhere. I love the North Summit Snowfield, but you need a transceiver, pole, and shovel to ski that and expert skills. The Big Couloir is a signature run with a section that exceeds 50 degrees -- and some sections that are relatively narrow (15-20 feet). As with the North Summit Snowfield, you need permission from ski patrol to ski the Big. But there's always Marx and Lennin which always give my sphincter a tinge -- 1200+ vertical feet of slope that is in the upper 40 degree range. I love steep skiing, and Big Sky has it in spades.
Don't overlook my suggestion to take some lessons; steep skiing is an area where technique is far more important than will. And if you decide to do so, ask for Ursula Howland, one of the best instructors (and my personal friend).