There is one Deadly Weapon for each type of weapon. The Seven Deadly Weapons are unlocked by completing special objectives, challenges, and missions. There is a video at the bottom if you need a visual of how to do each one. Credit to VGFAQ. Below the video is the time he shows each weapon just in case you only need 1 or 2 weapons.
The Seven Deadly Weapons are
Uriels Edge (Melee) You must convert 5 Marshaling Ground areas in New Hade.
Gallows Dodger (Pistol) You must kill 7 dexs after acquiring Shakespeare power.
Diamond Sting (SMG) Buy From Weapon store for $100,000.
Boom Chicka (Shotgun) You must open Blackbeard's 5 chest by finding 15 glyphs.
Armchair-A-Geddon (Rifle) First Weapon unlocked by completing “Take a Break” quest.
Ark of the Covenant (RPG) Defeat Satan's Top Agent by killing 10 Archdukes.
Last Supper (Special) Visit Dane at the 3 drivethru locations.
How to Use Weapons
Uriels Edge (Melee) To Use:
to slash hold
to shoot a fireball.
Gallows Dodger (Pistol) To Use:
to shoot and after X amount of hits you can
shoot an string of shots that are strong and fire automatically.
Diamond Sting (SMG) To Use:
to shoot with 1 shot head-shots and drops money
Boom Chicka (Shotgun) To Use:
to shoot and when fully upgraded its a one-hit kill on a legionaries and everything smaller.
Armchair-A-Geddon (Rifle) To Use:
to use Gatling guns and use
to fire missiles.
Ark of the Covenant (RPG) To Use:
to shoot and
to gain ammo.
Last Supper (Special) To Use:
to shoot frosting everywhere (Which does nothing) and
shoot explosive Frosting Balls.
Help on how to obtain them
Uriels Edge (Melee) When at the Marshaling Ground area go to convert a stone use Stone Blast and Ground Stomp to cover your back while you convert the stone.
Gallows Dodger (Pistol) While you roam around you will see a red circle on the screen representing the Dexs health bar and when you see it go straight to it because he disappears if you dont. but i finished the game killing 24 dex so there is not a set amount
Diamond Sting (SMG) Get $100,000 and buy the weapon. after obtaining all collectibles and finishing the missions i gained $2,300,000.
Boom Chicka (Shotgun) When you reach level 15 buy the collectible finder and collect all of the glyphs then open all the chest. (all shown with the collectible finder)
Armchair-A-Geddon (Rifle) I spent forever on this one. You must find a chair in front of a T.V. and sit in it. Its on the roof but NOT ON THE REALLY TALL BUILDING.
Ark of the Covenant (RPG) Kill people on the street and get a 6 Star Notoriety Level and after you kill the 10th Archduke you will fight Satan's Top Agent.
Last Supper (Special) When you reach level 15 buy the collectible finder and go to the 3 Drivethru locations
You just need 25 kills with each weapon. These can be boosted at a survival act by killing all enemies except to 2 or 3 and pausing the game and exiting the activity and restarting it or just kill stuff on the street.
If you have a better way to word something or format this Pm me
Hopes this helps pm me or comment if you have any questions or anything to add to the solution. If you give me a negative vote please comment or pm why so i can fix it.
0:00-0:33- Intro (SKIP)
2:10-3:14- Uriels Edge
3:15-4:04- Boom Chicka
4:05-5:01- Gallows Dodger
5:02-5:49- Last Supper
5:50-6:18- Diamond Sting
6:19-7:52- Ark of the Covenant
7:53-8:08- Outro (SKIP)