You should use ATA Secure Erase, it is much faster then most if not all the other methods. It generally works better on failing drives in my experience as well.

There are various ways to do it depending on your system/OS. Some BIOS have an ATA Secure Erase option built into them. (Parted Magic has a nice GUI, hdparm, etc.)

It has been a while since I used it on a super large drive, but I am guessing your 18TB will take a few hours.

Just make sure to select the correct drive (or have it be the only drive in the system preferable) as there is no recovery if you choose the wrong drive.