His goal as of current, is to prevent a case like that from ever coming to be.
That's why he needs to end the fight between him and Mammon as fast as he can before his presence gets discovered by nature and then sent to the spirit realm.
Spoon-feeding Mammon with more Mana is not going to help Orun avoid the outcome he is so desperately trying to prevent.
'From here on, I have to make it a physically oriented battle while making sure to prevent excessive contact with him" Orun thought, bringing his thoughts together and making his way toward Mammon who landed on the ground.
From how Mammon scrapped off Orun's mana with the black aura, and also how Mammon absorbed Orun's ray, it shows that the condition for Mammon using his ability has to do with touch.
He might be wrong, but there is no way he is going to neglect the first possible option.
Orun got down to Mammon before he could fly back up. He grabbed onto a nearby fallen tree and used it to whack Mammon's body.
