There is no way to know how strong Terrax is during a battle. Saying he has class 75 strength is kinda figuritive. We shouldnt over analyze what the writer was intending; we see that they want to make a statement by having Sentry beat up on someone who we dont know a ton about except that their tough (Terrax). The writer wasnt trying to tell us that Sentry simply has greater than clas 75 strength...He just wanted to make the broad statement that Sentry is powerful.