I think it is generally known that Mordor is the most solid overall faction. First, the Orc-spam is mostly a pain in the early-game since you have so little crowd control in that stage with most factions and you are spending resources on getting your economy going. Secondly, Mordor benefits from the snowballing effect. Thirdly, while you are throwing resources against the wall, Mordor replaces most of the casualties for free.
The majority of the factions cannot withstand Mordor in a direct slug-fest, especially Lothlorien and Rohan. Mordor's only real weakness that I can think of is their economy. Hit the eco, and they will have trouble getting out heroes, siege engines and Cirith Ungol units. Unfortunately, it is easier said than done.
Furthermore, I would personally push that Barricade thing to a 3 PP spell, but that would break the symmetry in the Spellbook, so I don't know what the team would say.
Frankly, any form of crowd control and AOE via Spells or Heroes is always useful against Mordor. Also, Archer spam can be useful, but it is risky and one-dimensional. I think Mordor needs some tweaks and Lothlorien as well, but what exactly, I find hard to say without shaking up the current ''meta''.
However, there is one tactic that I noticed is useful against Mordor (especially with Elves). Try to lure them into more narrow passages on the map, and engage them there. Mordor's army is the best when fighting like a big mass, often surrounding your units and whatnot, but if you take a chokepoint, you are largely nullifying their strength in numbers.
Edit: Oh, and what DJANGO said. I agree with that.