To my knowledge, cryptolockers target certain types of file (i.e. files with certain extensions such as .doc, .docx, .xls, .xlsx and so on). By way of contrast they won't be targeting .exe, .dll etc. simply because they don't want the computer to stop working, just encrypt YOUR data.
You could try creating your backup archives with "non-standard" extensions. In addition you could keep multiple iterations (e.g. a weeks worth of daily backups) AND keep them off-line.
Having said that, the cryptolockers will inevitably get more and more sophisticated. It's a matter of trying to stay ahead of the game.
Easeus Todo Backup Free allows encryption of a Disk/Partition Image Backup, to external drive, at the time of creating it.
Nasty and aggressive malware, to date, has targeted the common file types that it can see on a computer, synced Cloud folders, its local network and connected hard drives.
Theoretically, the scope of an attack could be widened to include all file types/extensions but it would take many hours to complete and would have difficulties of not being spotted and breaking Windows before completion.
However, the only truly safe way would be to disconnect the backup drive when not being used.