Since no one has done it yet, and the game doesn't explain it until afterwards, here is how you fight tengu.

Tengu will block attacks, and if the same attack is used on them enough times in a row, they will counter. Therefore, you need to vary your attacks. That means employ a mix of high, medium, and low attacks to prevent the tengu from countering, then damage it as much as possible when its guard breaks.

In addition, it is also possible to counter the attack the tengu attempts when it takes the initiative and runs up to you to attack. This is generally more difficult than breaking its guard.