With models like this I tend to assemble the head, torso, and legs as a unit. That gives a substantial piece to work with but with minimal occlusion from other parts of the model. The assemble arms, rider, steed, etc.
For some reason GW has the saddle/pelvis attached to the juggernaut's back with each leg separate. Sorta frustrating, but not a big deal. I plan on at least basecoating with an airbrush so things won't be too difficult.