Confused: The best move I found was to start a big Ko fight. Althought White playing at a would deny Black two eyes inside, White's outer group wouldn't survive long enough to fill the Black eye.
... or are very unlikely that they would.
Dansc: As we know, a six-point big eye produces 12 liberties. Counting the four white stones inside the eye, black still has 8 liberties - white, on the other hand, has only three. If white plays at a, black can play elsewhere five times and still win the semeai.
BarkOfDelight: Is this a fair problem for a beginner? The answer seems to be "White can't really do much." Is that the point of this problem? Just doesn't seem to fit the spirit.