How to break through the Gate of Oblivion on Jebus in week 2?

Could, for example, 1500 halberdiers be the main guard at Jebus Cross? Or 900 lizards?

No, the generator doesn't create more than a certain unit limit when creating a map. This limit is around 100, maybe 110-120. On Jebus, the Guardians of Order cannot be less than level 5, and even then, not all of them – only gr liches, minotaurs, pitfiends, and harpies.