This card basically lets a Seeker fight an enemy with their Intellect. If that were all it did, then it would just be a weaker version of Mind over Matter.
The card's only two saving graces are the card-draw (which is fine, I guess), and that it lets another investigator take the shot. The clearest combo is Shotgun, for which Expose Weakness hugely boosts the odds of scoring the maximum 5 damage. If you throw Double or Nothing in the mix, you could be looking at 10.
That combo is all well and good if you can pull it off, but it comprises three cards across separate classes and, realistically, at least two separate investigators in the right place, and a total of 7XP. That's assuming your scenario even has an enemy with enough health to make it worthwhile.
If that represents the best case for Expose Weakness, then it's in trouble. For the same 7XP, you'd be far better off getting Acidic Ichor and 3XP Emergency Cache to refill it, and dealing with the enemies yourself. As a Seeker, you're also in a much better position to make sure you actually draw those cards, using e.g. Cryptic Research or No Stone Unturned.
Also compare with "I've got a plan!" - the other major card built around weaponizing your intellect. It's more costly than Expose Weakness and doesn't work if you don't have the clues (although for a Seeker that shouldn't be a problem very often), but it deals solid damage and you don't need anybody else's help to deal it. It's genuinely unclear whether Expose Weakness even represents an improvement, which has got to be a pretty embarrassing comparison for the card that costs 3XP.
I give this card a rating of "unpurchasable."