1) Find the account you want to retweet from and create a RSS feed for it with a tool like:
TwitRSS.me - rss of twitter user feeds by screenscraping with perl
2) Create a Zapier rule to tweet from RSS
https://zapier.com/zapbook/rss/twitter/
Zapier is free but you have only 100 tasks so only 100 tweets free.
https://zapier.com/app/pricing