The Hierarchy are the most unique race in Universe at War. Unlike your typical RTS faction, aside from their main base structure, they do not construct traditional buildings. Instead, they call down enormous walker units from orbit that serve as both factories and war machines. Each walker has hardpoints that can be upgraded with armor, weapons, or production enhancements. This strength is also a weakness - precision-targeting specific hard points can literally bring a walker to its knees much faster than a brute-force attack.
Walkers' weaponry is crap for targeting large groups of small units, making them vulnerable to swarms of infantry. To protect against this, the walkers can produce Grunt and Lost One infantry units, anti-infantry Defiler micro-walkers (which can zombify enemy infantry with radiation attacks), light Phase Tanks, and genetically engineered Brute heavy melee units. An ideal Hierarchy attack will clear a path to the enemy base using smaller units, allowing the walkers to approach and blast the crap out of everything.
How to beat them:
1) Constantly scout the area between your base and theirs. If you can spot a walker approaching, you can attack and weaken it before it gets close enough to your base to deal real damage.
2) Attack their economy. The Hierarchy harvests resources from the environment with lightly-defended Reaper Drones, which wander the map sucking up resources as they go. They are expensive to build, so each one you destroy strikes a severe blow. Watch for them to stray away from defenses, and pick them off at every opportunity.