The Congress System has been roundly condemned by many hisorians as a tool of reaction.
Case is Past Imperfective since it began in the past and still continued to the past ..
Congress System is nominative.
Since System is feminine last letter is 'a'.
'by many historians' is dative since it answers by whom or by what.
'as a tool of reaction' is instrumental since it answers 'how'
Конгресса Система обршился осуждало по многые историкям как инструментом в реакцью.
система - feminine
историкям - masculine
инструментом - masculine