Summoners War: Chronicles

In the boss fight, when it comes to the point where you have to use the swords stuck on the ground to get thrown in the sky and avoid sword slashes, it works perfectly well till you get Friedrich's health low enough where it gives a notice saying that the phantom has betrayed him. After this phase, whenever you use the swords to jump and dodge the crescent sword slashes, the character fall down or acts as if getting back up from the floor immediately. This makes the jump fail and the boss heals and this goes on till he is fully healed. The boss is immune to all debuffs and heals around 20% of his health frequently because of this fail. How do we beat Friedrich now?