I've noticed that a lot of people seem to have trouble with Greavers and I think that the main reason is just that they are really hard if you don't know that they are coming at you. That's why I think levels with greavers need to have one of those yellow question mark things that you interact with on them.
I mean, wolvers and lichens get them and they aren't even that threatening and if you've never seen them before in tier two, there's a good chance you can get overwhelmed you're first time simply by not knowing that their aura penetrates shields.
Thus, they should one that says something like:
"Greavers are fast moving, hard hitting creatures of the Clockworks. Watch out for the haze unleashed when they attack as it inflicts status even when shielded. They are fragile however."
This way, people will at least have some idea when facing them for the first time and also, for others, it will serve to remind them to prepare and get the right equipment for the job like a shivermist or something.
Greavers are mixed in clockwork tunnels, there's no greaver exclusive level, like wolver dens are to wolvers.