Eh i highly doubt we could even gather enough Aether to even destroy Elidibus, especially with the issues that were already happening with the aether due to the First being at the point to cause a calamity. The Source was already getting an over abundance of light aether trickling in from the first. Y'shtola noted a oddness to the aether when she was activating that giant crystal for the wall. Urianger had noticed it as well and last but not least one of Matoya's brooms actually fell to the ground d. That caused her to say that she was having trouble with spells sticking.

Remember light aether causes stagnation. And at that point with Elidibus in Zeno's body in the bad future we may have been able to defeat him in the physical sense but most likely not in the sense of the aether.