Some good tactical mission scenarios have already been suggested, but some good ideas about scenarios in which they could emerge as a real and strategic resistance hope have also already been offered, such as them uniting various fractions in order to provide a more unified and coherent and effective resistance.
One thing I can think of is the political or military "turning" of various agents working for the chief bad guy (or his main commanders or advisors), so that at important moments his strategic operations, rather than just his tactical ones, are betrayed and/or sabotaged from within. The party could begin helping "to turn" the agents and operatives of the bad guy.
But whatever happens the party will have to move from being just a tactical threat and enemy and become a very real strategic threat and enemy, if they are to provide a real hope of winning the war itself, rather than just assisting with the winning of a few scattered and isolated engagements.
