Well, if the limiting factor for shapeshifting is your own mass, shapeshifting into something with a biological system like a lizard's tail automatically severing itself could reduce your mass substantially and quickly. Or just shapeshift into a cyborg, pull out detachable bits and then shapeshift some more.
As far as going larger than your own mass, well, that either can involve lots of cake or shapeshifting into something that can quickly digest/incorporate some other matter into yourself.
So, yes. Shapeshifting. Even if easy mass manipulation methods don't work, it still beats the hell out of summoning a specific entity unless said entity also grants wishes or somesuch.