With the permission of @Kelega from the Miskatonic Malcontent, I want to share this external link that breaks down in detail and with graphs on which traits to choose for every campaign up until TSK.

The author shows:

• What traits appear the most in each campaign
• What traits do enemies have in each scenario of each campaign
  • What is the best combination of 2 or 3 traits to cover most enemies in each scenario of each campaign
• What traits do treacheries have in each scenario of each campaign
  • What is the best combination of 2 or 3 traits to cover most treacheries in each scenario of each campaign

Warning This is full of spoilers, of course, but as Grizzled is mechanically made for guessing what the most common trait used is, if you have already been to this campaign and want to tech your deck, then this is all you're looking for.

For 6xp (Always Prepared and an additional Trait), you can either build yourself a repeatable enemy management tech or a 2-card constant +3 against treacheries.

Awesome for Patrice. Her signature weakness becomes trivial once you upgrade it to "always prepared"(still okay if you don't). The "monster" subtype trait is frequent enough for it not to be a dead draw. You can slam the extradimensional trait to have a juicy +5 on that particular test, or be content enough with +3 and put "humanoid" or "terror" on it to help your friends to deal with those pesky cultists/deep ones/frozen in fear.

With always prepared, you draw this card from the discard when you draw an encounter card, before applying its effects FFG answer: With the Always Prepared upgrade, you return a copy of Grizzled from your discard pile to your hand after you draw the encounter card but before resolving its effects. The intent is that you have the copy of Grizzled available to use for that encounter card.

It's a fairly expensive (in xp) skill card, so it's pretty easy to just avoid it. The fancy thing it does is 5 xp for Always Prepared, which is a lot even if you've got 2 copies, and it's limited to once per round. Usually it's pretty similar (+/- 1 symbol) to Unexpected Courage, and the traits portion limits it to monsters or treacheries, and can't be used to investigate or do tests on location cards.

Humanoid and Monster traits will get you a card that's useful against a bit under 100% of enemies, and enemies are only around 1/3 of the encounter deck. Adding Elite and Ancient One means it will give you 5 symbols against most of the more difficult monsters. The good news is, those 4 traits will get you almost all enemies (and double coverage against difficult monsters) in all campaigns for 3 xp. I'm not sure what combo's you would need to cover all of the tests in the encounter deck, but I can't imagine 4 traits would be limiting if you're careful.

Nemesis instead of Always Prepared (or with fewer traits; elite covers the monsters you'd want to use it on) sounds useful if you're willing to have a card in your deck just for fighting difficult monsters. Not a bad plan.

If you can find the traits that cover all of the really nasty treacheries, Mythos-hardened might be a way to thin the encounter deck. But that requires knowing about them ahead of time.

The basic or specialist-ized versions seem like they reliably give 3 or more wild icons almost all the time if you do a little planning and jumping through hoops. But Survivors have always been able to get lots of wild icons on 1-2 XP skills and occasionally on 0s. This one is solid but not especially exciting as far as that goes.

Mythos-hardened seems pretty bad outside of extremely niche counterplay situations. I rarely want to spend that much XP just to prevent one copy of one treachery from coming back, let alone limiting myself to a subset of traits and requiring a test and locking up an otherwise good card. "Fool me once..." and The Eye of Truth put this to shame.

Nemesis seems okay as multiplayer support (take elite trait for boss fights) or to enable Exploit Weakness, but I have little to say about it. No, the only really interesting part of the card, IMO, is Always Prepared, which lets you return one of these cards to your hand once per round if you draw the right kind of encounter. In one sense, 5 XP feels like a lot to spend on a card with no action compression at all, just to pass tests. On the other, a lot of decks do run multiple 2-3 XP assets that are mainly for skill boosting and test-passing. If we can actually make this trigger at least every other round, it might be worth while.

One nice thing about Always Prepared is that while Nemesis and Mythos-hardened only trigger if you actually commit Grizzled against the encounter card, Always prepared doesn't require you to be testing against the card at all. You get it back in your hand to do whatever you like with as long as you drew the right type of card. I believe it should even work when you draw weaknesses from your own deck, if the traits match.

This will be especially valuable if you have special incentives to commit cards to tests (Sea Change Harpoon), other uses for cards in hand (Wendy Adams, Cornered, Celaeno Fragments), or a tendency to draw extra encounter cards (Kicking the Hornet's Nest, Drawn to the Flame, Nature of the Beast, Nine of Rods).

I can especially see Patrice Hathaway taking this traited with "monster" and some common treachery traits like terror, hazard, or madness. She'll get one in the discard early and pull it back every time she hits a monster in the encounter deck, draws into her personal weakness, or fishes out a treachery, and she can just throw it into an invetigate or away for vioin money. If she misses she can fish with Drawn to the Flame and Nature of the Beast for more matches. "Ashcan" Pete can do most of the same stuff.

William Yorick can go tell others to "Let me handle this!" or take First Watch to make sure he gets his triggers in. (Calvin Wright also gets some of these options).

It's never going to be as viscerally exciting as firing up a Lightning Gun, Seal of the Elders, or The Black Fan, but I think it has enough value to be worth a shot.