well, how much time have you got?
pit traps work wonders
if you can find some sturdy trees or rock out-croppings, trip wires could certainly be effective.
anything to force a balance check (or multiple balance checks) to bring down a mech
failing that, any trip-wire trap could always pull the trees or rocks down on the mech
deadfalls could cause some damage, if not knock the mech over,a nd rolling a boulder down a cliff certainly ahs the potential to knock over a mech
mountainous terrain would be best, since you have lots of grades and valleys, cliffs, otehr natural features
making a blind to trick a smaller mech into a river or lake could be effective if the drop off is sudden enough
in fact, there's some really nice vision imparing spells at spell levels 4 and below
knock could easily be used to gain access to a mech that is meerly secured by locks
you could enlarge person in order to take on large sized mechs
hold person could temporarily stop a mech; long enought to board and capture it (most mechs are piloted by humanoids, so yeah