c2 = a2 + b2 is the third stage in Summer Lesson.
Those Guys are spawned in 10~16.7-second intervals.
Three B.B.Bunnies appear 10 seconds in.
Sir Seal appears after 30 seconds.
Use Titan Cats and long-ranged attackers like Dragon Cats protected by meatshields to defeat Teacher A. Alternatively, if you have Bahamut Cat, use him, as he can kill the boss in one hit. Many Uber Rares will also be effective.