Craven and its synonyms "dastardly" and "pusillanimous" are all basically fancy words for "cowardly." Don't be afraid to use them - here's a little information to help you recognize the subtle distinctions in their connotations. "Craven" suggests extreme defeatism and complete lack of resistance. One might speak of "craven yes-men." "Dastardly" often implies behavior that is both cowardly and treacherous or skulking or outrageous, as in this example: "a dastardly attack on unarmed civilians." "Pusillanimous" suggests a contemptible lack of courage (e.g., "After the attack, one editorialist characterized the witnesses as 'the pusillanimous bystanders'").