- Q: Do neutral cards count as a class for the purposes of Synergy cards? A: No. Neutral is not a class; it indicates a card that does not have a class. - Edge of the Earth Investigator Expansion Frequently Asked Questions section
Spirit. Synergy.
Fight. For each different class among cards you control, you get +1 and deal +1 damage for this attack.

This card is absolutely wonderful, even on hard, if you’re willing to sit on it a bit. Unless you’re Lola, you already control one class card, your investigator card. And with Edge of the Earth, there are a few of triple class cards that get you to 3. Double class cards are available, even for level zero. Also useful: having teammates who can play things for you (like Bob).
My favorite option is the investigator who can play cards from any class, as long as they heal horror. With Peter Silvestre, illicit alcohol, and the scroll of secrets(0), you’re there. (There’s also Shrewd Analysis, which is vital if you’re doing the ancient Stones trick. Easy!)
With your tableau set up, you can swing a +5/6 damage brutalizer for the extremely reasonable price of three resources. Overpower or promise of power can get you to ridiculous numbers. In duo, this can kill tough enemies. In a larger group, it can supplement your dynamite as a boss killer. And late game is when bosses typically show up!
Fear actresses and therapists, because they have many friends.
I'm not a big fan of the "by our powers combined" set, but I'll grudgingly give credit where credit is due, this is a good but expensive card.
3 Cost is steep, this is the major issue for this bad boy, but here's what you get:
First up, it's a fight action, so 1 damage base, that then means that with just 2 classes in play it's a 1xp, 3 cost attack at +2 for 3 damage. If you could take a 1XP Backstab with a +2 on it, people would most definitely do that. Already ignoring the whole setup thing of gathering all the classes, even with just 2 classes in play, the usual minimum a character can muster, this card is playable.
Everything on top of that is gravy. Do you manage to plop down a third class via a Dual class card? A fourth class through judicious dual class design or a triple class card? Are people trading cards with Black Market, Bob Jenkins, Teamwork or similar effects? The third class is relatively easy, Enchanted Blade by itself will get you a third class for most characters. Remember that your character is a card and it's included, so if a dude like "Skids" O'Toole plops down Enchanted Blade that means you've got 3 classes in play for an effective +3 and 4 damage and that right there by itself is gold. 4 Damage attacks are rare and powerful and worth the hoops to jump through for.
Can you max it consistently? That'll be rare. But a partial trigger for just +3 or +4 damage is excellent.