This is a hypothetical scenario following the stuff I've set up. If you want an exact strength for Broly at this point. He's around 1st form Frieza, just because Highschool DxD is a fairly weak universe in terms of anime.

Broly glares at Trihexa from a distance. This beast tried to harm his family. IT WILL PAY! Broly flies at the beast, shooting through the air as his hair starts flickering green. As the monster turns to him, Broly slams a fist into it's disgusting face, creating a shockwave as the beast is sent flying. Broly shoots a beam of Ki from his mouth, smashing into Trihexa and sending it through several mountains. Once the beast recovers, it turns it's several heads towards Broly and sends several streams of fire as it quickly regenerates. Broly flies around the streams of fire and launches several ki blasts while screaming in rage. "GRAAAGH!" He yells as an explosion of ki erupts from his body. He flies at the beast and fires several ki blasts at the beast, each one blowing massive holes in the monster. He forms a massive sphere of ki above his head and slams it into the Trihexa. The mindless beast starts disintigrating as it tries to heal. With one more monstrous shout, Broly seals the fate of the monstrosity. "GRAAAGH!" The explosion is heard around the world as the ki explodes, leaving a massive creater. In the center lies a small boy with 6 sets of wings. The boy's eyes open and he opens his mouth to speak a word before his body begins disappearing. Strangely, he shown no fear or anger. A tired smile Grace's his face as his head turns to dust. Marking the end of the Trihexa. Still out of control, Broly lets out a roar of rage and throws a large ki blast in a random direction, and it happens to pierce through Mars and blows a chunk out of the asteroid belt. Yasaka notices the beam of ki and teleports over to where Broly is. She quickly calms him down before she brings him back to Kyoto.

Um, I guess this is all I can use out of the reasons I gave. So, I guess this is the end of this fic. Be prepared for a more entertaining version of this idea, making it more in depth with character development and action