What comes to mind to make for a good skeletal structure would be a combination effort of pulley systems and compressed gasses, and to add tensile strength to the structure, a weaved mass of gas pockets that could be lined through the walls of the structure, where the legs are concerned, at least. The remainder would be using lightweight composite materials that have some durability to it, like carbon fiber/nanotubes.
At least, the idea that I have in mind is sorta based on how a king crab is built, of course, post-cooking them and eating them, and now I crave crab. Mmm, butter-based hydraulic fluid...
EDIT: (Weight/Balance)
To further spare materials and excess weight, we can honeycomb the endoskeletal parts (making a balanced framework to house the mechanisms), and plate the exoskeletal parts for external protection from the elements and such. We can then free up some pressure and weight issues so we can add a mighty powerful crane or two for properly weaponized pincers; of course, being structured similarly to be both lightweight and powerful (or at least, retain enough power:weight ratio).
EDIT EDIT: (Gases)
Liquid helium, being a bigger element, would be easier to contain, and unlike hydrogen, less dangerous if a breach were to occur in the structure, as it won't make the thing go the way of the Hindenburg. Plus, as an early-warning system, the crew will sound like chipmunks.
EDIT EDIT EDIT: (Addendum to above, and additional uses)
I think we can supply ourselves with liquid nitrogen as well as a means of further controlling and/of hydraulic pressures, further adding efficiency to pressure containment and such, but considering it would be on the excessive side, as far as I assume, for power, this system would work better for the pincer system, since using such a thing for weaponization especially, it needs the excess torque generated. But if anything, it would mostly be used to augment the containment system to supply the whole beast with adequate (amounts of) gasses.
EDIT EDIT EDIT EDIT: (Mobility and traction)
For mobility and traction, let's look at a spider's foot. If anything, they mostly walk on a series of hairs. We can essentially use series upon series of rebars or at least something similar to that that can be mass-produced as rods, but made of a lightweight enough material that isn't prone to rusting so easily, yet easy to replace and produce, as well as store for maintainence. Naturally, to accomodate for all-terrain purposes (as well as skewering anyone unfortunate enough to be stepped on), we'll need to make all the rods/ribs various lengths, and projecting outward from the straight-most rod center-most on the feet. This can also minimize the square footage of contact, allowing for more precision when moving through cities and such, while possibly minimizing what gets stuck to it's feet as it travels. As long as the structural composition of the rod-materials is similar to how a tree is structured (carbon nanotubes can mold well for that, while being rustproof, but won't be cheap; and if properly molded, should allow for enough structural flexibility for compression reasons on-contact with the ground, while hopefully still being structurally sound enough to be durable. The idea for reinforcing the legs in a jaw-breaker like pattern from core and outwards (cylindrically), can be a good concept start for the solid rods that make up the feet (straights and coils for several hundreds/thousands of layers to make up the rods)), they should last a considerable amount of time, while each one is supporting each other.
I not only want this big sucker to walk across land, I want it to climb mountains, and span across canyons. This should help that be a possibility while still working towards our end-goal.