Ashwatthama plans to kill the Pandavas

Ashwatthama along with Kripacharya and Kritavarma then left in the night for the Pandava camp. They groped their way and then sat below a tree. There they saw an owl devouring the sleeping birds. Seeing this Ashwatthama said - This is the way to kill the Pandavas.
Kripa said this was not the correct way to kill an enemy. A sleeping enemy was not to be attacked. Duryodhana brought about this condition by not listening to advice, and you are doing the same now. You want to defeat the Pandavas, ask for a war and fight. If you are not convinced, let us talk to Dhritarashtra and see what he says.

Arjuna fights Kripa

Arjuna then asked Uttar to take the chariot near Kripa. Uttar took the chariot and Arjuna blew on the conch shell, Devdutta. Kripacharya blew his conch and then showered arrows on Arjuna. Arjuna cut those off. Arjuna started using divine weapons which scared Kripa. Arjuna cut off the bow of Kripa and his armor. Kripa picked up another bow which Arjuna cut off.
Angry Kripa hurled a weapon at Arjuna which was cut off.
Arjuna then cut off the four horses of the chariot and killed the charioteer.

Taking a mace, Kripa jumped and that too was cut off by Arjuna.
With nothing else left to fight with, Kripa left the battle field.

Kripa

Bhishma seeing the princes growing up, he chose Kripacharya as the person who would be able to teach the princes the art of using weapons.

Janmajeya wanted to know why a brahman like Kripacharya was adept at using weapons.

The sage continuing said :

The sage Gautama had a son called Sharadwan who born with Shara (arrow) in his hand.

When he went to the forests to meditate, such was the force that Indra tried looking for ways to stop his meditation. Indra sent a beautiful apsara to tempt the sage. The sage was distracted and the sight of such a beautiful woman made him lose control. As he was a great saint, he still managed to resist the temptation and controlled his desires. But his concentration was lost, and he dropped his bow and arrows. His semen fell on some weeds by the wayside, dividing the weeds into two - from which a boy and a girl were born. The sage left the forest for another place.

The king Shantanu happened to be in the forest and he saw the children there. He brought them over and took care of them. When the children were old enough, Sharadhwan appeared there one day. He blessed Shantanu for taking care of his children with love and named the boy Kripa and the girl Kripi. He taught his son the art of weaponry. Soon Kripa was an expert and kings and princes from far started coming to him to learn. At a very early age, he became known as Acharya.

Bhishma seeing Kripa diving his time between other external students and the Kuru princes, started thinking of getting a new teacher. He thought of Dronacharya and asked him to teach the princes.