(Maybe if you remember how nuclear work and step to make it, and maybe you remember tactics that our world create and use in world war. Maybe then you can be useful.)
As said earlier in the thread, nuclear physics is insanely difficult. Not only that, you'd need a foundation of molecular and quantum science to even begin the calculations necessary for proper enrichment, radioactive decay, neutron production, criticality and hundred other details needed to make a bomb work. That's assuming this world even has the scientific advancement necessary for such things. Also, this war feels more like WW1 than WW2, and the science of the nuclear bomb developed around the time of WW2. So even if all other things align, which is unlikely especially given the magic, I doubt the universities and professors of this world have the background for nuclear physics. Even in WW2, this was considered cutting edge and required billions and the sharpest minds of the age to make it work.
What would be interesting is trying to create a magical equivalent of a nuclear bomb. Perhaps easier given the fact that magic here runs on mental thought and skill rather than the physics of this world. Several options spring to mind:
- Simple explosion magic scaled up, efficiency is questionable if not optimized
- Alternatively, a magical means to create immense pressure difference at a short period of time, for simplicity. Goal is to create a powerful shockwave without need for explosives, which might make the spell easier
- Simple kinetic spells. Meteor strikes are possible, so perhaps we can accelerate a sufficiently dense artillery round to immense speed by way of acceleration magic, or Orion-drive style explosive spells on the base of the round
- Concentrating mana into intense energy density, then detonating/releasing it. If there is an electromagnetic field equivalent for magic, use that to increase yield and efficiency. If that is the case, using plausible fusion reactor designs would be ideal. If not, default to magnetic field equivalent for better simplicity, if poorer yield
- Wide range but simplified transmutation spell to convert atmospheric gas to a toxic variant, an easy target is to convert carbon dioxide to monoxide, or oxygen to ozone, assuming magical chemistry knowledge is sufficient to pull this off.
- Mental/Psionic magic to demoralize and/or drive enemy combatants insane. As this will be a weapon and not population control, simplicity will be prioritized over finesse. Hopefully this will make the spell easier and stronger.
- Curses, but scaled up. This is questionable though, given the very specific rules and sacrifices needed to pull off curses.
- Good old fashioned necromancy. Raise your dead soldiers. An additional ID-protection system to prevent enemy necromancers from raising your dead while still be able to raise yours is optional, but recommended
These are all off the top of my head after 10-15 minutes of light speculation. The presence of literal reality-altering powers really opens up avenues for more creative means of murdering your fellow men for pointless reasons.
However, given the fact that 1) None of these seem present, and 2) Meteor spells require dedicated Archmages, implies that magical development is not on the level required for scaling up offensive spells. Thusly, the options I stated aim to use (hopefully) simple versions of spells, with only range and perhaps strength scaled up. There is also the concern of mana resources, which seem to be expensive and limited. Thusly, simplicity again.
TLDR: Nukes may be not be an option, but magic, creativity and some knowledge of modern science can make effective alternatives
