Recently for a question all the answers (correct) were downvoted for no reason, I just marked them flagged for moderator's attention, Is there any other efficient way to take some action on such activity ?
There is no reason to flag answers for moderator attention because they've been downvoted.
Downvotes are anonymous and can be cast for any reason a user may choose. Moderators cannot and should not reverse downvotes based on their own opinions. The job of a moderator is not to decide whether an answer is "correct" or not—that's the job of you and everyone else with the reputation to cast votes! Moderators should only be flagged in cases where their intervention is required.
The only situation where downvotes would be reverted is when they are associated with a pattern of abusive behavior, such as in the case of serially downvoting a particular user. And in that case, there is an automated script that runs periodically to detect and reverse this type of abuse.
If you come across a post that you don't think deserves to be downvoted, then you should upvote it. Eventually, community consensus will decide the appropriate score for that particular contribution.