Isshin, the Sword Saint phase 2 no spear
After phase one, continue to attack before he transforms until he parries. Once he parries he goes into basic retaliations from his first phase. If you are good enough and he doesn't manage to get too far away from you, you can keep on him and keep continuing to force him into block stun and eventually a parry until you get another deathblow.
If at any point he gets far enough away from you or there is a lull in your assault he will do the animation to go into his spear (and lightning form for phase 3).