The Divine Planet is surrounded by raging winds and unforgiving storms and there are only a few areas where the intensity of this hellish weather is slightly toned down. This 'gap' isn't nearly enough to fit the whole undead army but Arthur wasn't worried for he knew that no one is foolish enough to attack him in the middle of this disadvantageous environment.
If Katrina is planning to attack first, the best timing would be when he's entering the Divine Planet and slowly guiding his army to his unimaginably big piece of land.
The White Specter Clan may not have many people but each and every disciple is strong. Furthermore, there are a considerable amount of elders who reached the Overgod Realm. There's also Katrina, who's an Exalted God.