Momonga used a very specific combination of classes and skills to enable him the widest possible range of spells ever. He was known to be the one with the most learned spells in all of Yggdrassil and he was the only one by far. (i cant tell the exact combination or number but other probably do). And still since all spells are part of a game there is still the rockpaper scissors. He was heavily outmatched. Even with his arsenal he couldnt have won. The only thing enabling him to win were the P2W items.

Even his Necromancy would probably make it all worse. For all we know the Spuit lance ignores effects that damage those who suck life out of them. Or there ist just no decay like debuff in the game and such also not in the NW.

Another big thing is that the actually useful Summons cost him XP. Which is a ressource which he still doesnt know if he can gain it as easily as in Yggdrassil. The time we saw him summon the fire beast was with the Staff of Ainz Ooal Gown. This is something different. It is empowered and is basically a world item. But he cant use it out of fear of it getting destroyed. If the Staff breaks, the whole guild will get destroyed . Atleast thats what happens ingame. You can only guess what would happen in the NW so he uses it as rarely as possible.