A Strongman, is one of seven strong demons, that prefer to remain hidden or undetected. The one in the spotlight will be the spirit of Jealousy. The Word and the Lord[YHVH] uses the word jealousy in a good sense; this Strongman is in a bad/evil/negative sense.
Christ Jesus in Luke 11:24, and Matt.12:43 speaks of a demon that has been ejected, and returns with seven other spirits, more wicked than himself. We must identify what "kind", or type of spirit we are dealing with. The more serious ones would like a human, versus a pig.
The Strongman is quite familiar with a person's blood-line; usually involving three  generations of people that did Not properly deal with this spirit. Christ ask "how can one enter a Strongman's house, and spoil his goods, except he first bind the Strongman"? We have been busy swinging at gnats, and taking a whipping from demons.
The spirit of Jealousy = is extreme envious. The malice when meditated on creates enemies, and indignation. It can spill over into murder. Pride and Jealousy is what got Lucifer/Satan a death sentence in the first heaven age. Lucifer was to guard the Mercy-seat, but when the Father read his mind, Satan received a swift kick to the seat of his britches; that is how we, in the Name of Christ Jesus should respond.
Obtain the Mercy-seat-only by murdering the very One that created him. That to is his wicked plan for those whom know that they are made in the "image of Christ". This can be either spiritual or physical, it doesn't matter to Satan. Bind this Strongman, in the name of Christ Jesus: Rebuke him, and eject him. Our words have power, and your Angels are standing by LISTENING.