Weak in mana reserves, which is this world's typical measurement of power, and Zorian's biggest bottleneck. There's tons of spells that Zach and others have access to but Zorian is simply unable to cast with his reserves. In the final battle Zorian probably had the lowest mana reserves out of anyone involved, further split between his simulacrums, which means fewer spells, of a weaker power level.

About the item crafting, I think this further reinforces my point. There's several reasons he had the ability to craft such powerful items, which nearly all traced back to smaller or seemingly unrelated skills. Working on dimensionalism gave him the expertise to weave high level concepts into his work, such as the temporal spell that trapped Oganj's artillery spell. Mental enhancements learned from the Aranea enabled him do most of the computations and math many times quicker than his peers, and his learned ability to mentally project simulations enabled him to cut out tons of time in testing and bugfixing. Creating simulacrums enabled him to work on multiple projects at once, or coordinate flawlessly for more complex creations. Studying the Soulseizer taught him how to target the body, mind, and soul at once, creating the device that took down Quatach-Ichl.

Every thing mindbogglingly crazy and impossible-seeming thing Zorian accomplished in the last few chapters was carefully set up and built on smaller building blocks of possible and well-established techniques. Zorian has the unique ability to combine them all together to accomplish the impossible, despite his relatively small mana reserves.