Tactics for playing against Sylvan.

2 OptiMAL
I think that against Nature, you should take monks rather than crusaders.

I don't think so.
You take monks. So what? Nature mainly plays offensively because there are many flying units like mantises, griffins, fairies, and phoenixes... They'll see your monks and that's it.
Nature doesn't really play defensively, as it's a bit weak. It doesn't have units that can effectively counter crusaders and angels.
Sandstorm... I don't think it will significantly affect the army. The army will move evenly, and it will be able to withstand any attack from "outside."
Against water creatures, Order's Obelisk is a good choice (it drops often), and you don't need to worry too much about arrows. An Archangel is almost always available (in my case, it only didn't drop 1-2 times, and the rest of the time it always did) (I'm talking about Equinox). Therefore, you won't suffer much from a hero with ranged attacks.