1 Dispel or 1 Push in the final qc phase will end your spread of plague very quickly.
True, but I think Plagued will best be used liberally. One of them isn't that threatening, but 3(or even 4) of them together are (granted, you are tempting a Destroy Magic at that point). Even if only 2 creatures are getting hurt per turn, it still has the same efficiency as Ghoul Rot (2 damage per turn for 6 total mana). And if the enemy mage responds by breaking up their swarm, that's still a success (since it will hopefully cause them to lose momentum, as well as making it easier to pick off individual creatures).
What's more, it should cost you significantly less mana to kill creatures than it costs to summon them. For example, a timber wolf costs 9 mana to summon, and possesses 10 health. Assuming 4 of them are sharing a space, that's 36 total mana investment. Casting (and activating) 4 Plagued uses 24 mana. Assuming the wolves don't scatter, the curses aren't dispelled, (etc.), all of the wolves will be dead in 3 turns (for all intents and purposes, 2 turns if you reveal all of the curses at the very end of a round). That's obviously not a full-proof strategy (nothing should be), but it can be pretty effective.
The real downside of Plagued is that it effects
all creatures in the zone, including your own. That is interesting, since it means that it is a curse meant for the Necromancer, not the Warlock. Plop down some zombies, throw out some Plagues, and sit back as your zombie horde chases down the scattered enemy forces.
What's a Fortress of Death? High Guards are only effective, when they use their special ability ... they have to be on guard for this and you need to attack them. So don't. Their normal attack roll with 4 dice is not so scary without their special ability, right?
I see it like this: Guardian Angels (as well as Panzerguards) are great at wasting time. They are hard to kill, and generally force you to commit 2 creatures to an attack instead of 1 (of use some method to temporarily get rid of them, like Force Push). But they aren't really threatening. A counter-attack of 3 dice is only meaningful against something with low health and no armor. If you have to attack them, it's annoying but not usually life-threatening.
Temple High Guards, on the other hand, are guards which can actually murder stuff. If you attack one with a small or medium creature, it is going to suffer heavily (and if it dies, it doesn't even deal any damage). The Temple High Guards do suffer a bit from their lack of Intercept (and I'm not sure why they don't have Reach, considering they are using halberds...), but they are still useful for defending stuff against melee creatures (even if they can't go toe to toe against the really hard-hitting stuff).