Parashurama and Bhisma started the battle, and Ganga arrived trying to stop Bhisma from fighting against his guru, but was unsuccessful. The battle went on for 23 days and finally on the 24th day, Bhisma wanted to use the most powerful weapon, Prashwapastra.
When he was about to use this Prashwapastra, Narada, the divine sage and the Devas, interfered and said that the use of this weapon can cause huge destruction. Parashurama and Bhisma concluded the battle and Parashurama declared that it was a draw.
He then met Amba and narrated the events, hearing this Amba decided she will avenge against Bhisma herself by performing severe austerities to Lord Shiva. Saying this Amba started doing vigorous penance at the banks of the river Yamuna, and she became extremely thin but still stood firm to destroy Bhisma. Later, she performed penance in the river Yamuna, and continued her penance for 12 long years, which even ended up burning the heavens.