IIRC I completed mine from a normal wasp enemy (like those in Reapers March, not like a miniboss though a few of those exist too).

These trophies are a pain to get, for me the last one was from a Wamasu.

Try going to Entila's Folly in Auridon, its the delve on coast line south of Skywatch or east of Mathiisen wayshrines, visit the bottom right room on the map, and you will find a lot of wasps. I got mine from there very quickly indeed, good hunting!

> I also need Nereid, Wamasu, and Strangler, so I've been farming a lot in S. Shadowfen and around Brackenleaf in Grahtwood, because all three can be found in both those places.

Heh heh, I'm standing at its mouth now. It's the Nereid Temple, and the quest is "Right of Theft," which is picked up from Nara just south of Verrant Moras wayshrine. It's one of the zone quests, but it only takes a few minutes to get far enough to enter the cave.

For folks searching in the future, this guide is good:
https://alcasthq.com/eso-monster-trophies-achievement/
but here are some additions/clarifications

• Magnificent Bat Pelt (Giant Bat) also drops from Werebats (?), found near the Woodworking Survey in southern Greenshade, near the southern coast. Easy to get a route where they spawn as fast as you kill them.
• Nose Shackle (Ogrim) can be farmed in the Tower of Lies in Coldharbour. Head NW to the Tower from the Hollow City wayshrine 'til you find questgiver Gwilir. You don't need the quest, but jump down into the pit. There are 12 Ogrim there in a circle around the bottom of the pit. They respawned at exactly the rate I could make it around the circle.
• Mammoth: Mammoth Ridge in The Rift. I found going east of Riften ran me into player werewolves leveling their skills, so this was much less populated, and had a larger number of mammoths. I found a circle-8 path that had 10 or so mammoths that respawned fast enough.

I got giant bat, wasp, strangler, and orgrim yesterday, just have nereid, mammoth and wamasu left. I suspect wamasu will be my last, too.