The Juggernaut is a...
As per patch 1.4, Juggernauts are much more damaging and use their attacks and vortex abilities more often. They have gained a shockwave ability to do a short range knockback with strong damage that will hit even players behind them. Juggernauts are also no longer hackable.
Armed with short range dual machine guns and long range mortars. It's easy to kill them because they turn slowly. Just keep their back in front of you to avoid their machineguns and launchers.
Although this looks like a fearsome beast its actually does much less damage than the Predator that flies over it- which must be defeated first if you want to stand any chance at beating it.
The Juggernaut is unique in that it must be facing your direction to fire (as most other enemies can just fire at you even if they are facing the wrong direction), furthermore its turning speed is laughable, allowing you to literally run rings around it without it being able to shoot back.
If this isn't easy enough you can actually totally avoid the Juggernaut by following the path to the west at the ATF base and circling behind it. The walkway just beside the main door has a break in the fence allowing you to sneak past it, kill the overseer (who actually holds the key to the door) and enter the research center before the Juggernaut can even turn around.
Massive robotic machines that are approximately 5 meters tall and can weigh up to 60 tons.
Dual Energy Canon, Mortar Launcher, Machine Guns
Juggernauts have a vortex attack (similar to the Demolitionist Skill Reality Ripper) that sucks enemies inward towards a central target point.
Juggernauts are extremely heavy, and therefore are slow moving.
Mid to Long Range
Because of their size, Juggernauts are limited to open battlefields or large structures that can accommodate their size.