To activate trials you must
pause, go to
options and select the
Trials sub menu.
For this particular achievement you must make sure you have the
"Rub some dirt on it" trial turned on. This modifier makes it so
"All healing potions heal only 1 health".Hivernal is found in
Emprise Du Lion. It is a level 19 dragon and can be found in the first "Colosseum" building you come across over the bridge. (The bridge is located in the middle of the map, to the west of Tower camp, and is unlocked via a War table operation.)
This can be done on casual to make it easier.
It can also be done by reloading an older save if you have one. Or at least I was able to do so, I had the trial on before fast travelling to EDL. (No idea if that matters, I would assume not but figured I'd mention it in case someone has trouble with a loaded save.)
If you do not have a save you can go back to and have already killed it, you will have to do it on a new play through.
I've added a video by VGFAQ which has the location of the dragon at the start just for anyone who needs reminding/has no idea where it is.