You can find them on some of the Skellige islands, but they are not common.

I found some north of Hanged Man's Tree, on an island with a crumbled tower.

Just south of Mudcopse or whatever the name of that town is, on the way to that elven mage cave I encountered them twice and just ran like hell (level 4).

I killed some when I was level 5, but that's only because I found that Puppet Axii works almost every time on them.

Just fought a group of them ( guarding some loot ) for the first time in some early area in Velen. Level 4 character as well, so it took a while... and lots of Igni kiting.

Also tried the crossbow, and it did an amazing 1 point of damage. :(

I'm going to more or less avoid them for now, at least in large groups. If it were only 1 or 2 I might try to tango with them though, but I encountered like 7 of them and just put Roach into turbo mode to sail past them. :) I've got the Temerian armor set and a silver sword that does up to 125 damage I think. I don't know what those creatures are vuln too though as I don't have them in the bestiary yet. I imagine I will soon though as I'm battling like 342342 wraiths at the moment so I'm sure to gain a level soon hopefully. :)

Yeah, the crossbow has been less than useful so far. It's more like throwing a nerf soccer ball at someone. I think you need to get custom crafted bolts to make it useful, I have 3 different types and they all suck. I mainly use it to let creatures know where I am and lure them to me.

I found the crossbow to be amazing underwater. Kills enemies twice my level in two hits.

at least in early game, crossbow is super useful for 2 things: shooting underwater, & shooting harpies. other than that tbh i havent explored it (im lvl 14 just got cat school gear the crossbow isnt till lvl 25 or so, so i think its maybe a late game weapon)

yup here only spot ive run into them... 8 baby 1 warrior

There was an abandoned castle filled with them, but I can't remember where... It's in Velen, near Oxenfurt if I remember right.

Go south, there is either big tree guarded by them (but there are few), somewhere in the left part of the map, near central boggy area, but it's very long way, or go south-east (bottom-right), also in hills just south of bog, west from Nilfgaardian military camp..Levels 9-11 IIRC.

The regular, smaller Endrega were OK, but there was also a bigger one ( warrior? ) which took a while to defeat. Most creatures still seem to kill me with a few attacks, so I'm relying a lot on bombs, signs and heavy abuse of the dodge maneuvers. I'm also still going with style over efficiency at this point, so I'm not even using the best armor I found, since I still like the look of the first armor set. :P

Using the crossbow to pull enemies and take them on one at a time is a good strategy, I'll have to try that. So far I haven't used it much outside of the griffin fight. Are those custom crossbow bolts unlimited as well, or do they work kind of like bombs?

I got a crossbow from Vesimir in White Orchard when fighting the griffon.
Crap, I forgot I even had bombs, and I just fought a gargoyle that took FOREVER to kill. I bet bombs would have kicked it's ass too.
The default shitty crossbow bolts are unlimited. Any other ones in the game have a count on them in your inventory.

I think he meant a stronger crossbow, you find later in the game?

So you have to keep crafting more of the special bolts?

And bombs are pretty fun. Just threw a grapeshot bomb at a group of roadside bandits earlier, and it splattered half of them all over the place. :D