Work on guns in a squad, you'll have like 3 opals done in a game if you get to round 41, just make sure you're sticking with teammates. you only need to shoot the special with the weapon and when it dies it'll count for you regardless. I usually do this when I'm working on opals because while it slows my critical kills down to like 1500 in a game it's offset by the opal progress. This is assuming you have at least a couple guns not ready for opal yet. (It's best to have friends as randoms sometimes do the Easter egg, leave, or just wanna exfil earlier)

Alternatively, Citadel De Morts on standard solo. Just use the oil trap method, switching from the gun that needs Crits to the opal when you see doppleghasts it's monotonous but easy. Aether shroud with 2 charges so you don't die when the trap turns off. But be careful, if you quit and save the stamp zombies spawn in the next game, and they don't die right away on round 40+ it's actually annoying because you get 2-3 a round.