To unlock the Pugnus Sanctus Dei, you need to complete two of the Hidden Histories around the map. They're both in the city so you'll find them on the way towards All Mine , but if you want to grab this weapon as soon as possible complete the following:

Once you've finished these and unlocked the weapon, head to your safehouse or a Friendly Fire and equip it, it's classed as a melee weapon. Now just find any car, you'll probably want it to be a parked car so the driver doesn't escape, and just press and/or to punch it until it explodes for the trophy.