Opportunistic Gain is a Wonders of the World Achievement that you can obtain by watching the infighting between fungi. You must essentially trick the hordes of fungi under the Jadeplume Terrorshroom's cave into attacking each other.
How to watch the infighting between fungi in Genshin Impact – Opportunistic Gain Achievement guide
Start by heading to the Jadeplume Terrorshroom's cave west of Vanarana, either via the Teleport Waypoint or the portal in the sky. Once inside, you'll notice that the clearing housing the boss is elevated, and you can fit between the plants to plunge deeper into the cave.
Once you're at the bottom of the cave, you'll notice various fungi grouped by Element. You must lure as many fungi as possible toward the Floating Hydro Fungi. Approach the different-Element mushroom enemies or attack (but not kill) them, then lead them toward the Floating Hydro Fungi. You must keep doing this until there's a good number of different-Element mushroom enemies surrounding the Floating Hydro Fungi.
The field will get chaotic once the fungi are bunched together as they'll all try to attack you, but what's important is that their skills will eventually start affecting each other. In particular, the Floating Hydro Fungi's bubble can trap fellow fungi. So simply wait for an unfortunate fungus to get trapped inside the bubble to get the Wonders of the World Achievement called Opportunistic Gain.
Check out our guide on How to get The Smell of Grilled Mushrooms in the Morning Sumeru Achievement in Genshin Impact to unlock another Achievement in the same area.