>Well, let's see. nadir is the opposite of peak, so "even for a native snadirer" would be the opposite of "even for a native speaker". If we want to convey the same intention, we have to negate "even for native snadirer". So, either, "even for a non native snadirer", or maybe "not even for a native snadirer". Or perhaps "even for a native non snadirer".

Or "uneven for..."?