After winning the battle on the beach against the minor Boss that resembles a giant crab and you can move onward to the next area, save the game, but do not proceed ahead. Instead, backtrack to the beach and defeat the enemies modified with the Mashogane. They will not stop respawning, and will provide an unlimited amount of Mashogane to boost your weapons attacks to their maximum at this point. Kill off a good number of them, then when a large number of enemies are present, use your Kamui to take them out. This may also give you a healing item such as a Miracle Pill or Tengu Herb. Note: Retreat back to the save point whenever your health gets low. Do not use your healing items as you will need them for the Boss fight that is just past the save point near the Soul Gate.
