Critical Hit clarification

[Clipper]

It's discarded after it's used. This is because it's not an Item, nor is it any other card type said to stay in play in the rules, and it does not have a persistent bonus at the top of the card.

Only Items require the "Usable Once Only" text if you are meant to discard them after using them. Other cards are always discarded after they are used unless the card or the rules say they stay in play. Some of these will still include the "Usable Once Only" text, but it is unnecessary. This part of the rules covers cards like Critical Hit:

Originally Posted by Rules

Other Treasure cards (like Go Up a Level cards) are “specials.” You may play these at any time, unless the card itself says otherwise. Follow the card’s instructions, then discard it, unless it has a persistent bonus like an Item.