Actually, Flyrants are Nids only answer to multiple knights. 4-5 flyrants will take them down without too much hassle. Devourers can whittle them down, as can the haywire template. Warp blast can punch them, and onslaught lets you change your fire angles to get around their shields. One Flyrant won't do it, but multiples from different angles will get through and take them down at a decent rate. Frankly, I don't think we have many other options against them, which is part of the reason why flyrant spam is so prevelant.