When you earn an elemental mastery, a message appears saying, "Elemental Master! You finished your opponent with all your HP's, your reward has been increased" [sic]. There are two mistakes in this: First, "HP's" should be "HPs" because it is plural, not possessive. (Note: A similar error is common when referring to years—"1900's" is incorrect; use "1900s".) Second, the comma after "HP's" should be a semicolon because it is connecting two independent clauses.
--Grammar Hammer!