So you could not use a PeCo spell on an armored knight because you can not see the Corpus part?
And it gets worse if you try to use a R: Sight Mentem spell.
maybe with a creo prequisite you could make spikes to cut into victims as they fall into the pit?
Yes, you could, because the armor is considered a part of the Individual knight for spell targeting purposes:
If the armor is (for targeting purposes) part of the person, then seeing the armor is equivalent to seeing the person, even if the armor is opaque and covers them seamlessly.
This targeting equivalence is, however, apparently one-way - the armor (when worn) is a part of the knight, but the knight is not a part of the armor, so a PeTe to destroy the armor doesn't need to be T:Part. (I can't find this directly stated anywhere, but, e.g., Doublet of Impenetrable Silk is T:Individual, but does not require that the affected garment not be worn when the spell is cast.)
And thus, snow can be considered part of ground beneath as well. As there is note of moss on boulder.
Depends on how much snow