Diego Gonzalez and captain Paolo Goltz scored the goals for Lanus on Thursday as the Argentine club completed a 4-2 aggregate victory in the Copa Sudamericana semi-finals.
Lanus will face Brazil's Ponte Preta in the final of South America's second-tier continental club competition.
Having won by a goal in the first leg in Paraguay a week ago, Lanus stretched their aggregate lead early in front of their home fans with Gonzalez striking in the 12th minute.
A long ball forward was headed down to the midfielder and his first-time half-volley from the edge of the D rocketed in off the underside of the crossbar.
Libertad entered half-time 3-1 down in the tie but started the second half strongly with Jorge Gonzalez scoring from a direct free-kick in the 54th minute as his shot went under Lanus' wall and beat the hosts' goalkeeper at his near post.
But just four minutes later, Lanus dealt the decisive blow with Goltz converting from the spot after Jorge Pereyra was tripped in the box.
Ponte Preta are the final team standing between Lanus and their maiden Copa Sudamericana title.
