Investigators who have problem with resources can make a use of it in a multiplayer game. Roland or Harrigan can put it in a 0 xp deck when they start a campaign. Τhe thing is that you dont want to be overwhelmed by enemies so usually the maximum will be +1+2 for the attack. Don't get me wrong but you want to deal with the enemy as fast as you so the Knife from core is better option than this one with the discard option.

This is a very strong spell for mystics in multiplayer games and the more people you are playing with, the stronger it becomes. It is similar to Dynamite Blast except that it only deals 2 damage and requires you to succeed at a Fight test, though you can choose the weakest enemy at your location to fight. Those drawbacks are balanced out by the fact that it costs 2 less resources and is in a class that is substantially more likely to have spare resources to play it. Additionally, it synergizes with other popular mystic cards such as Arcane Initiate and Uncage the Soul. Finally, and possibly most importantly, it is much easier to arrange multiple enemies at a location that contains investigators than an empty one where Dynamite won't hurt your fellow players.

Storm of Spirits is especially good in Sefina Rousseau decks, where it is possible to play Storm of Spirits two, or even three times in a single turn, clearing a location of even tough enemies. And for the truly bold, it can be combined with Double or Nothing though I haven't personally tried this combo yet.

Guard dog is most effective at taking out weak enemies with 1-2 health that deal 1 damage. This allows it to maximize its (guaranteed damage) triggered ability and save a few actions.

Take this example in a Roland deck:

1. Weak enemy (1HP) engages Roland in a location with clues.
2. Roland has guard dog in play, and decides to draw a card (or just walk around like a boss).
3. This triggers an attack of opportunity (AoO), and damage is assigned to guard dog.
4. Guard dog triggers and the enemy is defeated.
5. Roland's ability triggers, and you discover a clue.

Effectively, you've killed an enemy and discovered a clue while using an action doing something else entirely.

Clarifications

-Timing structure (for the above example): trigger action is declared (i.e. move) -> AoO triggers -> damage is assigned (not dealt) to guard dog, triggering its ability -> enemy is defeated -> Roland ability triggers > damage is dealt to guard dog -> trigger action is resolved (move).

-If guard dog has 1HP remaining, it can be assigned both lethal damage and horror (1 damage + 1 horror) from an attack.

One of the single best upgrades you can buy for your deck. The ally slot is arguably the strongest slot available to investigators and being able to have two or even three allies out at the same time will dramatically change how you build decks.

There are allies so good that building decks that don't play them is unthinkable if those allies are available, such as Leo De Luca for Rogues or Dr. Milan Christopher for Seekers. Playing Charisma allows you to look at the other allies available to these investigators and consider including them as well.

Even better though is the health and sanity buffer that your allies provide you with. Having three allies out at the same time makes you virtually unkillable to any non-direct damage/horror. I would always consider getting Charisma and playing an extra ally before picking up something like Bulletproof Vest or Elder Sign Amulet if you are worried about trauma.

Very strong card...in very specific decks. You are only going to want to include this in your deck if you have either A.) A really strong ally that you want to get as early as possible AKA Leo De Luca or B.) You have disposable allies that you play for a one time effect such as Research Librarian. The strongest disposable ally is The Red-Gloved Man , which you could play to either grab two clues, or fight an enemy twice, and then play Calling in Favors to pick him back up to play again later while replacing him with another ally from your deck.

The problem with this card is that you are really going to want to play at least 5-6 allies in your deck, as you need to have one out first to play it, and then enough left in your deck to have good odds of finding one in the top 9 cards. Some investigators just aren't going to want to play 6 allies in their deck due to which ones are available to them, and the resource costs of those allies. Plus, you'll have to get at least one copy of Charisma to avoid being stuck with allies you can't play in your hand.

On top of that, you are going to run into situations where Calling in Favors is a dead draw, such as when you haven't got an ally out yet, or when you already have your best allies out and don't want to replace them. I'd probably only play a single copy of Calling in Favors most of the time, unless you are trying to abuse The Red-Gloved Man.