I have a few problems with what I'm hearing, Riaddon. First of all, the Plasma Rifles on your Crisis suits don't have nearly the range of the Destroyer's Gauss Cannons, thus can't target them unless the Destroyers are foolishly close. The same goes for Heavy Destroyers. Your only real option there is Missile Pods, and those aren't as dangerous to Destroyers as their Gauss Cannons are to your Crisis Suits. Furthermore, Crisis Suit Plasma can't damage a Monolith at all. In fact, unless there is a new piece of gear in their codex that I'm not aware of, no weapon of the Crisis Suits other than the Fusion Blaster has any chance at all to damage it, and in the case of the Fusion Blaster, chances are still slim.
Having played against Tau many times, I'm going to have to say that Destroyers definitely aren't easy for them to deal with, no more than Immortals are. The only way you're going to easily put the Monolith away is with Railguns, not Crisis Suits. Have you made a mistake, and said Crisis Suits when you meant Broadsides?
At any rate, if you phased out these Necron players by turn two, I'm going to have to say that the player was...well, either severely under points, or extremely inexperienced. That is assuming that both of you were playing entirely within the rules for the Tau and the Necrons, and weren't minsconstruing any of them. Wwe had a player at my gaming store who made up rules for Tau, cheating more blatantly than anyone I've ever seen, and his results were close to what you describe; thus it's possible that if the rules are misread that the Tau could have this sort of efficacy against the Necrons. As to Necron rules being misread, that happens all the time, though such misreadings usually make them stronger, if anything. It just occurs to me that the Tau army didn't have the fire power to do most of the things you describe last I checked the new codex. My friend has the new codex, though, so if I'm missing something(having not gone through the entire thing), please give me some page numbers and I'll go do some research.