Imagine that the adversary have several accounts. He can build different blockchains with different sequences of blocks. Every branch will have different cumulative difficulties. The attack will be successful only if he manages to find a branch with difficulty higher than the difficulty of the legit chain. It's a problem of finding an extremum (optimization). None of the methods (gradient method, etc.) except exhaustive search over all possible combinations could be used for that coz Nxt uses SHA256. So we get classical "find the nonce a-la Bitcoin" game here.