Start a game of Horde on beginner as Jack with bots, or a friend, and build a forge as quick as you can. You can build it right away if you start at a higher wave in the horde options. I usually start on Wave 18 or 28 to get some of the scion weapons. Here's the list of weapons you need to convert.

Boomshot - Dropped by Scions
Breaker Mace - Dropped by Warden
Buzzkill - Dropped by Scions
Claw Light Machine Gun - Dropped by Elite Drones
Cryo Cannon - Dropped by Scions
Dropshot - Dropped by Scions
Embar - Dropped by Snipers, or it can be purchased by Fahz
Enforcer - Dropped by drones, or anyone playing Del
Gnasher Shotgun - Dropped by enemies, or anyone playing Marcus, JD, or Kait
Hammerburst - Dropped by enemies
Lancer with chainsaw - Dropped by anyone playing Marcus
Lancer with Grenade Launcher - Dropped by anyone playing J.D
Retro Lancer - Dropped by anyone playing Kait or purchased by Marcus
Longshot - Dropped by Snipers or anyone playing Fahz
Markza MK1 - Dropped by enemies or anyone playing Fahz
Mulcher - Dropped by Scions
Overkill - Can be dropped by enemies, a teammate playing Del, or purchased by Kait
RL-4 Salvo - Dropped by DR-1's
Torque Bow - Dropped by Hunter Elites
Tri shot - Dropped by Stumps, Guardians, and DR-1's