Here are some help-full backup commands which could help back-files with specific backup criteria also known as Backup Specified files.
Backup Archive logs
BACKUP archivelog all NOT BACKED UP 1 TIMES;
BACKUP archivelog COMPLETED AFTER 'sysdate - 3';
BACKUP archivelog ALL NOT BACKED UP SINCE TIME 'SYSDATE-3';
Backup database and archive logs from x days ago.
backup format '/backup01/backupsets/%U' as compressed backupset filesperset 50 database plus ARCHIVELOG not backed up since time 'SYSDATE-3';
Backup to Roll Forward Standby Database using increment backup from SCN
If the standby database apply gap has widened considerably and a large number of archives have to be applied, an incremental backup from SCN can be taken and applied directly to the standby database to quickly apply the changes and make it current.
BACKUP INCREMENTAL FROM SCN 9654987 DATABASE FORMAT '/backup/incStandby_%U' tag 'INCSTNDBY';
You may also like Rolling Forward Standby database with missing archivelogs.