What Orun is counting on by relentlessly attacking Mammon is to get it to exhaust its mana.
Of course he does not expect it to be easy when he considers that what he is dealing with is a Demon lord.
Mammon is sure to have a lot of mana, an almost seemingly supply of it in fact if according to its title as a Demon lord is indeed true.
He lacks the ability to calculate the specific amount of mana or threat that the current Mammon poses, but the fact that this is not its real body is a plus one for Orun.
In exchange for not knowing the original threat level of Mammon, Orun decided to believe in his own threat level.
He is strong, and has proven that to be true on several occasions. So long as he knows what his strength and weakness is, he should have no problem confronting any enemy.
Besides, as Dilya had earlier stated, the current form of Mammon is not its real form.
It is a form manifested through the use of the mana that it has to spare from its essence.