The idea is to find out how many times tweet was retweeted from the particular user and then from the people who retweeted it and so on and find the user whose followers shared tweet N times first.
Say threshold is 100.
Alice shared a tweet and got 20 retweets.
One of the Alice’s friends - Bob has more followers and his retweet was retweeted 100 times.
Bob was first whose retweet was shared 100 times. He’s the target of the search.