The Overmind ubermech. Armed only with a beam cannon under it's single eye and some guided rocket pods on the shoulders, is it nevertheless feared by Resistance forces everywhere, for one reason and one reason only: the raw power of the thing in melee combat.
It has no melee weapon. It doesn't need one.
It just smashes units apart with it's bear hands, picks them up and tosses them over mountains or simply crushes them underfoot like treading on snails.
The Resistance's only respite is the fact that, due to weight limitations on things that size, adding armour would probably cause it to collapse under it's own weight, forcing the designers to leave the inner mechanics somewhat exposed. Whilst this does give it a chillingly skeletal appearance, it also means it's not invulnerable: put enough firepower into it, and it will go down.
God help you if it's got support units though.
Technical notes: It's fully articulated. The crotch and shoulders both rotate around their points on the spine, giving it a quite human gait when walking (or at least that's the idea). The 'boxes' on the shoulders are the rocket launchers, mainly there for anti-air purposes. Have fun! Oh, and also really, really tall. Figure we might as well make use of the non-spherical hitboxes...
The Battery Replacement Helicopter for the Mobile Positron Siege Cannon Walker. Flies in, inserts battery, flies off, pure and simple. It's by far the easiest way to reload the Siege Cannons, and certainly the only way to do it in the field outside of a specialized hanger, but the Helicopter can still be shot down. The pilots for these 'copters are well known for their expertise in evasive maneuvers and ECMs, but the death rate among them is still notoriously high as they make prime targets for anything and everything that can shoot at them, preventing the Siege Cannon from firing again. It's not uncommon for more than one Helicopter to be dispatched to reload a single Siege Cannon, in the hope that at least one will be able to get through.