In deliverance ministry, legal grounds is shorthand for an area of agreement, conduct, or spiritual involvement that may leave a person vulnerable to ongoing deception or oppression. Scripture sometimes uses the language of a foothold, a snare, or giving opportunity to the enemy.
The phrase does not mean that Satan has authority equal to God or that freedom is earned through a perfect formula. Jesus has already triumphed over the powers of darkness. The ministry goal is to bring hidden things into the light, remove agreement with lies, and stand in Christ's finished work.