Juggernauts are your units to use after heavy soldiers. They are stronger and have much superior hp and damage, however they take up 2 unit spots so you can't mass produce them. They are stronger when in Humvee's or in Jeeps, combined with the speed of these vehicles and the power of them together in the same vehicle is a formidable opponent. You can produce them in the Fort, the fort can also produce important things like Artillery or transport.
When up against Juggernauts, they can be overwhelmed through Tanks or lots of Turrets combined with the firepower of a Headquarters or Command Center. They are good at taking care of stragglers left behind from fighting by Artillery as they take a lot of punishment and can defeat single heavy tanks quite easily.