A nice Oracle Database 12c New Feature enhances the multisection backup, introduced in 11g: You can use it now for image copies also!

Multisection Backup with Image Copy

RMAN> report schema;

using target database control file instead of recovery catalog
Report of database schema for database with db_unique_name PRIMA

List of Permanent Datafiles
===========================
File Size(MB) Tablespace           RB segs Datafile Name
---- -------- -------------------- ------- ------------------------
1    347      SYSTEM               YES     /u01/app/oracle/oradata/prima/system01.dbf
2    235      SYSAUX               NO      /u01/app/oracle/oradata/prima/sysaux01.dbf
3    241      UNDOTBS1             YES     /u01/app/oracle/oradata/prima/undotbs01.dbf
4    602      USERS                NO      /u01/app/oracle/oradata/prima/users01.dbf

List of Temporary Files
=======================
File Size(MB) Tablespace           Maxsize(MB) Tempfile Name
---- -------- -------------------- ----------- --------------------
1    40       TEMP                 32767       /u01/app/oracle/oradata/prima/temp01.dbt

RMAN> configure device type disk parallelism 2;

new RMAN configuration parameters:
CONFIGURE DEVICE TYPE DISK PARALLELISM 2 BACKUP TYPE TO BACKUPSET;
new RMAN configuration parameters are successfully stored

RMAN> backup section size 301m as copy datafile 4;

Starting backup at 29-JUN-15
using channel ORA_DISK_1
using channel ORA_DISK_2
channel ORA_DISK_1: starting datafile copy
input datafile file number=00004 name=/u01/app/oracle/oradata/prima/users01.dbf
backing up blocks 1 through 38528
channel ORA_DISK_2: starting datafile copy
input datafile file number=00004 name=/u01/app/oracle/oradata/prima/users01.dbf
backing up blocks 38529 through 77056
output file name=/u02/fra/PRIMA/datafile/o1_mf_users_bs2v934z_.dbf tag=TAG20150629T180658
channel ORA_DISK_1: datafile copy complete, elapsed time: 00:00:15
output file name=/u02/fra/PRIMA/datafile/o1_mf_users_bs2v934z_.dbf tag=TAG20150629T180658
channel ORA_DISK_2: datafile copy complete, elapsed time: 00:00:15
Finished backup at 29-JUN-15

RMAN> host 'ls -rtl /u02/fra/PRIMA/datafile/';

total 616468
-rw-r-----. 1 oracle oinstall 631250944 Jun 29 18:07 o1_mf_users_bs2v934z_.dbf
host command complete

We use backupsets by default now especially also upon the DUPLICATE DATABASE command, which leads finally to image copies of course.