Windfall is the rare card with no downside: draw 2, no target, no tempo cost, and nothing anyone can do about it. Play it the moment you draw it — earlier is strictly better, because the two cards it finds become options for the same turn. In a deck of 30 Attacks with very little refill, it is one of the few ways to actually restock a hand.
Best with Huang Yueying (Ingenuity): Windfall is a trick, so she draws 1 more — three cards from one. Its Heart faces also feed Guan Yu (Martial Saint), who can spend any red card as an Attack.
There is no direct answer — it cannot be responded to. The only counterplay is to take it out of the hand first (Steal / Dismantle), or to starve the draw phase so it never arrives.