Shrink the file, but dont use truncate only.

You will need to ensure that the reuse desc is set to nothing to shrink it down as low as possible, otherwise it will only shrink to the last active part of the log.

Remember, a backup be it a full / diff / log will not shrink a file, shrinking is an expensive process and causes lots of issues especially around index fragmentation in the MDF. The file has grown to that size as it needs to be that size to handle all the transactions you have within the 30 minute backup window.

If you set the log backups to 15 minutes, you will potentially end up with a smaller log, but all depends how big the file needs to be to handle 15 minutes of transactions.

Read the article on managing the transaction log, and size your log accordingly.