Persona 4 true ending adachi dungeon i already beat the mini boss in sixth level but i dont know how to get past the sixth level to the 7th level someone help plz plz

There isn't a 7th floor in Adachi's dungeon.
Once you beat the mini boss on the 6th floor of Magatsu Mandala, you have to return to Magatsu Inaba's second floor, the floor where the huge hole on the ground was. Almost at the end of the map there was a sealed entrance which you couldn't get through, once you have beaten the mini boss on the 6th floor, you can. There's a save point there before you face Adachi.