PHOTOS - delete on iPhone but keep on iCloud

That's not possible, since they are synced to iCloud using iCloud Photos, and if you delete pictures from iPhone they are automatically deleted on iCloud too.

The only procedure you can use is to transfer your pictures to a computer:--> Archive or make copies of the information you store in iCloud - Apple Support

When done, you're sure that pictures are safe on your computer, so you can delete them or some of them from your iPhone.

Regards

Giulio

You can't.

Delete from the device also removes the iCloud copy.

Turn off iCloud Photo Library, then delete photos. If you'll turn it on, they'll come back to the phone.