Shusaku Game #19

In this game Shusaku played against his teacher, his teacher took back the handicap advantage and never lose it. Key battle started over right top corner-White managed to took the corner from inside then attack the Black wall from outside, when the Black retreated, White's group over lower right also joined the hunting and got stronger. After that Black group final out, White took the sente and started to invade the bottom. Black tried but could not do much against the invading White group down the bottom, instead White took over the whole Left side-Black had chance to start a ko down the lower Left corner, my question would be was that Black's only chance and could Black win the ko? consider White prepared a couple big ko threat down the bottom.