YOU SAID:

At this point the tall boy had already asserted himself as the one leader

INTO JAPANESE

この時点で、背の高い少年はすでに自分が一人のリーダーであると断言していた

BACK INTO ENGLISH

At this point, the tall boy had already declared that he was a leader

INTO JAPANESE

この時点で、背の高い少年はすでに自分がリーダーであると宣言していました

BACK INTO ENGLISH

At this point, the tall boy had already declared himself a leader

INTO JAPANESE

この時点で、背の高い少年はすでに自分をリーダーと宣言していた

BACK INTO ENGLISH

At this point, the tall boy had already declared himself a leader