Board Thread:Wiki Discussion/@comment-30183199-20161009145047/@comment-27701762-20161009193407

Toriel's specific claim is:

"You could have gone through the barrier after you got ONE SOUL.../...taken six SOULs from the humans, then come back and freed everyone peacefully."

So as Pancry said, the idea would be to go out and either absorb six more SOULs from dead humans, or forcibly take those SOULs from living humans, then return and break the Barrier once he has a total of seven SOULs.