How do you get strongest monster on dragon quest monster?

Rhapthorne 2 isn't the strongest monster you can get but he is still pretty strong. He is rank X. You get him by synthesising zoma and rhapthorne. To get zoma you need a wight king (tortured soul, king slime) and a malroth (buffulorge,demon-at-arms). To get rhapthorne you need a nimzo which is made by a drakulard (drakularge, pazuzu) and a malroth. You also need a dhoulmagus which is made by a psaro (ruin,rose guardin) and an alabast dragon (drakulard,mechan-o-wyrm)