The masked man hadn't planned to use this card until he was assured of victory, but while he was growing stronger through learning how to actually utilise Divine energy properly, the truth was, the Herald still held the largest advantage. His reserves of energy were massive and were only continuing to grow because of the effect of the Undeath Class which the masked man had imposed earlier.
Such an advantage was further emphasised by how he had nearly ended the battle with his last imposed Rule; the absolute frost. If the masked man had been a second late in countering with his own Rule which kept him safe from the cold, things would have gotten pretty bad.
That said, on the matter of Rules, the masked man had noticed that they had similar limitations to Creeds, the major one being that if the gap in power between the user of a Creed and his target was obscure or worse – in the target's favour – it was hard to affect them directly.
Rules worked the same way.