Other than leveling them using a ship that is no higher than they are by 4 levels. (So not using the monty if any are below 97).

In future, rotate your sailors I guess. Keep the BO, Gunners and scout onboard at all times, rotate the rest of the sailor so only the lowest are onboard and follow the rule of not using a ship that over the level of your crews lowest sailor + 4.

But easy and cheap? Not that I can think of. Just level them. What level are they?

Start another BO and go the other BB line.

You get to see the rest of your nations BBs and the supports should level that much faster (assuming we have the same perception of "extremely underleveled").

I did it with my KM crew and the lowest sod is at lvl 106 while the remaining crew is between 117 and 111. 

You leveled to nebby with 5 supports that low? Ouch. You didnt do yourself any favors by using them on a ship 50 levels higher, they would have incurred a 99% exp penalty.

Well, good luck with that.