An enemy weakness that asks the question "why do mob goons cause horror?" Looking at the two elements, the effect and the discard condition, we get:

The effect: Daniela clearly has some history with these guys, and so she knows how do deal with them. About the only special thing about them is that direct damage effect, which turns off things like Bandages†, Guard Dog, Peter Sylvestre (2) as well as soak in general. The horror component of their attack is more worrying for Daniela, since she has less Sanity to use, but these bad boys aren't likely to get more than one hit in before she finishes them off, so it's not that significant. As hunters, this isn't an enemy weakness you can ignore, and Daniela isn't big on evasion, so the "mitigation strategy is doing 3 damage in one action. The obvious plan is to use Mechanic's Wrench to provoke an attack, then defeat them with Daniela's ability and a wrench attack. It's so obvious that it's a little odd that they don't have 4 health just to mess with it. If you'd rather not take an attack, many weapons + Vicious Blow will do the trick, as will Chainsaw. Brute Force might be the most direct solution, as Daniela would attack at 8 vs 3, making the 3 damage pretty likely to land, even without extra boosting. If you are worried, throw in an Overpower and get a card draw. I'd also like to endorse HanoverFist's janky solution, above. It's not efficient, but very satisfying if you can pull it off..

The discard condition: Beat them like rented mules.†† It's likely to take Daniella 1, maybe 2, actions to finish them.

Like most enemy weaknesses, this is below average, unlikely to end a scenario or campaign or, really, cost you more than one action.

† Thanks to Susumu, I realized that I meant Flesh Ward, not Bandages. Mea culpa.

†† Note: this should not be taken as an endorsement of beating mules, who are mostly hardworking and affectionate creatures.

Somewhat comparable to mystics' Seal of the Elder Sign which costs 5 exp. Both practically gives you an autosuccess.

Seal doesnt work as well if your stats are low and you need to put additional resources to succeed (arguably Preston Fairmont and sometimes Calvin Wright) but gives you the benefit of the investigator elder sign.

Stroke of luck also has a small chance of failure but can be used after revealing the token (and always gives +1 skill when commited).

In terms of the value of the elder sign, a lot of survivors have situational abilities that can be hard to quantify. Perhaps it's a moot point since (as far as I can recall) only Agnes Baker can take both cards. In her case, her elder sign just gives lots of stats. You'd be paying 5 exp just for an autosuccess where stroke of luck would only cost 2.

For most other investigators on average, the elder sign also gives you a 1.5 card advantage (like Mandy Thompson Harvey Walters, William Yorick, Silas Marsh, Father Mateo). 1.5 cards is worth about 2 exp (according to Emergency Cache and Lucky!).

So with seal of the elder sign you're paying 3 for the autosuccess and 2 for the side benefits. With stroke of luck, you're paying 2 for selective autosuccess after revealing a token (maybe that's worth 0.5 or 1 exp?).

In most cases you're paying only 2 exp for a around a 4 exp effect is a pretty good value exp wise. I think it might even be worth buying and rebuying for the last 2-3 scenarios of a campaign.

An enemy signature weakness, designed to annoy Trish. Looking at the two elements, the effect and the discard condition, we get:

The effect: There are three ways to deal with this weakness, the mug's way, Trish's way, and Trish's other way. The mug's way is to fight them; that is likely to be 2, maybe 3 actions, although another investigator can always do the fighting for Trish. Trish's way is to evade them. As a , Trish has a lot of evade options. The entries above give some good suggestions, and Slip Away, Slip Away (2), Stealth, and Manual Dexterity (2) can help with the test. Trish's other way mostly involves boosting investigation checks and using Obfuscation to ignore the AoO (or just eat the damage; Trish has 8 health) and relying on Trish's special ability to evade them. The real secret of the Shadow Agents is: why are they hunters? If they get discarded, they aren't going to hunt, and there aren't too many cards that move an investigator without bringing the enemy with them.

The discard condition: Evade them or kill them.

All in all, this is a below average signature weakness, like most enemy weaknesses.

Another enemy signature weakness, with some Erich Zann flavor (let's ignore that Zann played a violoncello, not a violin, and Patrice should properly be hauling a chair around). Looking at the two elements, the effect and the discard condition, we get:

The effect: This is a pretty interesting card; while it is in hand, Patrice only needs to succeed at a fight or evade test (importantly, you do not need to do 2 damage, merely hit). StyxTBeuford, above, gives a very good rundown of the various approaches and options. Additional tools Blur (1), Sword Cane, Brand of Cthugha (1). As usual for Patrice, a copy of two of Cornered, plus skill cards like Promise of Power and Last Chance can help make that crucial test. Importantly, the Peril and Hidden keywords only matter if the Watcher is in hand, so, if the test is a failure, other investigators can come to Patrice's assistance. 3 damage is no joke! The most significant effect of this card is that it effectively reduces Patrice's hand size to 4 as long as it isn't addressed, and that reduced size really affects her ability to dela with this weakness.

The discard condition: Succeed at a fight or evade test while it is in hand or kill the damnable thing.

All in all, this is an average signature weakness, below average once Patrice has built up some XP. It might get knocked up a level in Hard or Expert, where failure becomes more likely.

An enemy weakness with a Mateo twist. Looking at the two elements, the effect and the discard condition, we get:

The effect: It is a relatively weak, although mildly persistent enemy which also grabs the out of the chaos bag. If Mateo is going to fight them rather than endlessly evade, he has a few options, some better than others, with the 3 health of the Serpents being the significant factor. Shrivelling and Azure Flame will do the job reliably, but at the cost of 2 actions and 2 charges, which is steep. The 5 XP versions will do it at half the cost, but they are pricey solutions with their own dangers. As noted above, Spectral Razor is an excellent option. So is Radiant Smite, if you are leaning into (sadly, no Holy Spear for Mateo). Brand of Cthugha (4) is another option for a one action kill, with Brand of Cthugha (1) not terrible (about as effective as the "basic attack" spells in actions and charges). A friendly combat-oriented fellow investigator is, of course, the cheapest option, but not always available. The sealing effect is thematic, but not especially troublesome. Entire scenarios go by with no draws, after all, and anything that takes the token out of the bag (e.g. The Codex of Ages or Crystalline Elder Sign) denies the Serpents their due. The main effect is to make evade even less attractive as a solution, should the get sealed.

The discard condition: Kill them and be done. Like most enemies, any investigator can do the job.

All in all, this is a below average signature weakness (maybe average if it comes out early in solo), like most enemy weaknesses.