I'm running the quest Wild At Heart trying to kill the werewolf, but once I get it down to a certain point in health, it begins to heal at a ridiculous rate. Anywhere from 21-50+ per half-second or second, whatever the game's units are.

Aside from just coming back later with more gear, which I'd rather not do, how do I prevent it from healing? Or can I even?

It seems you need to hit it often enough so he can't start healing. Use IGNI + sword attacks.

Also, check out bestiary, might be it says something about him.

Video:
https://youtu.be/dwlLw6ZCve4

Pretty true.

I ended up having to use a large combination of things to my advantage. Prep the fight with a combination of Cat, Thunderbolt, and Tawny Owl potions as well as Quen. Taking out the smaller wolves was a must, then block and dodge until whatever health you lost comes back either via foodstuffs or your stock regen.

I found that striking three times, then jumping back to wait for a counter attack worked the best. Once he gets between 1/3-1/2 health, he'll do a long howl to start the healing process. Immediately consume another Thunderbolt to open with some strong attacks until the animation cycle is finished. Then do as many short attacks as you can, using either Quen or Aard to prevent return attack.

Worked for me! Thanks for the help.

Depends on your level. If you are on equal level or lower on higher difficulties you have to use bombs that will turn of his regeneration. Vampires are the same so watch out and always be prepared for upcoming fights : ) You can overcome his regeneration with DPS but you have to be flawless wth combination of igni, dodging and striking. GL HF

Read my answer here:

https://www.gog.com/forum/the_witcher_3_wild_hunt/how_do_i_beat_werewolf

It's a quest made by some lowbrain anyway... No checkpoint on the way so if you screw up the final fight, you start from scratch.

Thank you for highlighting the checkpoint part, I'll make sure to remember that.

KILL IT WITH FIRE!!! XD No really, igni, slash, slash igni, slash, slash, quen, dancing star. Repeat. lol

I don't remember having any trouble at all with that fight, so . . . was he modified in a patch?

Just save it yourself then. I had no issues saving the game before going to fight him.

Anything he is vulnerable to will delay regeneration.

(Cursed Oil, Fire (Dancing Star/Dragons Dream/upgraded Grapeshot/IGNI), MoonDust Bomb).

You can also just whale on him. If he is regenerating at 25-50 DPS, then almost any competent equipment and skillset will do far more than that. Quen to protect you from the worst of his first attacks, and hit as often as possible, dodging to the sides or short backwards to evade his stronger attacks.

I don't recall seeing any dip in vitality during this fight at all, and I did nothing special. Just got close and hit him with my upgraded fast attack build.

When you have a lower level character it is really important to collect as many places of power as you can safely reach.

I forget exactly how many there are, but I found a few which I don't think are in any of the online guides.
There are certainly 6 in WO, 9 (or 10) in Velen/Novigrad and 8 (or 9) in Skellige (though some of these aren't easily accessible until later in the game).
Being level 6 with 5 skill points is very different from being level 6 with 19 skill points.

What really helps is this alternate Igni mode where Geralt emits a continuing spray of fire.
Put enough rune stones into your armor/boots/gloves and most fights will be much more simple.
Depends on your fighting preferences, but you can bolster up any sign with using those runes.

Baldars Gate had the same problem with trolls,you knock them down
and they start regenerating and belt the shit out of you.The only way
to finish them off was after they were knocked to the ground was fire
magic.

Actualy if you haven't done that so far. Buy better silver sword. You can buy it from the very same Blacksmith you question about Hana (the lost wife).

AND I'm not sure if i dreamed it, but during the fight don't kill the wolfes, if you do werewolf starts to regenerate.

Viper sword does poison damage. This will mitigate his healing. Fire will stop his regen. So many choices... It was easy once I got to level 7 or whatever the quest recommended. I was too low a level at first. Best news. there's a marker RIGHT outside his hut to fast travel back later.