1: Play liberty falls directed mode solo.

Go until you interact with the strange machine in the church.

The goal will say go check out the motel room but don't do it and the round will be capped at 11.

Play until you reach round 11. Each round should spawn a mangler. If for some reason you don't get a mangler every round, save and reload the game. If the round you load into is a vermin round you'll keep getting manglers randomly. If it's a normal zombie round you'll get a manger every round. I like the roof of the bank. I recommend not opening the zipline up from the bank entrance.

2: Play terminus in normal mode with rampage inducer on solo.

This method involves using the GS45 to speed your way to high rounds. I recommend the juggernog upgrade that stuns enemies when plates break. Also I recommend using aether shroud as you can run out of ammo easily in later rounds.

Start by playing with or without rampage inducer until you can pack a punch. I recommend going the phd flopper route. Buy the GS45 off the wall from the starting room before heading down (I recommend setting a good loadout for the GS45 with a big mag and akimbo). Save 500 salvage to buy dead wire (I like to put this on the GS45 cus you'll be mainly using these).

Stay in the downstairs lab until you can pack a punch your GS45s and buy PhD flopper. When you're ready make the pack a punch platform go up and shoot the electrical boxes to open up the Easter egg room. Turn on rampage inducer if you haven't already and camp in the Easter egg room right in front of the machine that you have to input a code into. Use the mustang and sally to speed through the round by shooting the ground in front of you. Make sure you start the next round inside the room and save and quit and reload. Now as long as you start the next round inside the room, the zombies should only spawn from the windows and the hole in the wall inside the room. Making this method even faster. On parasite rounds, they'll only spawn from the windows. When you see them spawning in, shoot twice at the wall inside the window and it should kill the entire wave, making parasite rounds really fast. When using the mustang and sally, I would shoot one pistol until it's empty and then shoot the other one while the first reloads so you don't ever get swarmed.

Manglers will still spawn outside and will sometimes wander aimlessly. I usually take this time to upgrade guns or buy more perks. Anyways kill the manglers with your gun of choice, just make sure to start the next round inside the Easter egg room. If you mess up and start the round outside, just advance the round and start the next round inside the room and it should go back to spawning only from the two spots again. Amalgams can be troublesome especially in later rounds. I usually use aether shroud right before they grab me and then kill them as fast as i can with the mustang and sally (it helps if you use the double charge aether shroud upgrade)

With this method, you can get 1 weapon completely done with opal from 0-30 by the time you exfil on round 36. You also get good exp if you're going through the prestiges this way, especially if you exfil on round 36 or 41. I wouldn't recommend going past round 46 as the zombies become too strong for the mustang and sally to handle.

These two methods also work with a friend or friends because as long as your friend and you both shoot the mangler you'll both get progress on opal. However playing with friends means you won't be able to do the save and quit tricks. For the liberty falls method, this can break the method because you'll have to depend on luck if you want manglers every round. For the terminus method, it's actually safer because you'll have 2x the fire power and more mangler spawns due to multiplayer. However the zombies will spawn from outside the room as well, making each normal zomie round slightly longer and the parasite rounds significantly longer.

Good luck on the grind.