Make em glide baby
True flight with a large creature is hard in principle, since at a certain weight animals simply can't generate enough muscle power for sustained flight, since it requires even more muscle power, which requires more mass, which then requires more mass. Large animals like Pterosaurs cheated the system by having hollow bones, which allowed them to circumvent the mass problem while still being quite large.
So perhaps these Shenrons instead utilize a similar flight system to leaping lizards, where their ribs fan out to form web-like wings for the express purpose of gliding. they would not be true wings, but it would allow a form of flight for these massive serpent dragons.
They could launch themselves from high places in the same manner as Asian flying serpents (by forming a J shape), perhaps from cliffs or enormous trees, and glide down rapidly towards their targets, diving at them and wrapping themselves around their target like an anaconda.
Rather than giving them the same flight system as leaping lizards, you could give these Shenrons multiple sets of wings like those of a wyvern, which would allow it to not only grab onto trees, but allow it to glide without it being too awkward of a system.
Image sourced from The Genesis of Species - 1871
Info about Flying Asian snakes: http://blogs.discovermagazine.com/80beats/2010/11/23/video-reveals-how-a-flying-snake-slithers-through-the-air/#.XXHi1PZFyDs