Federico Insua kicked things off with a 32nd-minute opener and Sebastian Dominguez added a second before the break.
It was all over when Mauro Obolo made it three and, when Roberto Brum was sent off (57) it was clear Banfield were beaten.
Ivan Bella then made it four to round off a fine night`s work for the leaders.
Olimpo were equally impressive, seeing off Belgrano Cordoba 4-1.
Grenddy Perozo put them ahead in the 20th minute, but Belgrano equalised through Cesar Manzanelli`s penalty on the break. They then had Claudio Perez sent off and the floodgates opened after that, with a second-half brace from Andres Franzoia and a goal from Nestor Bareiro wrapping things up.