The leaders of the horde would utilize tactics and strategy based on their knowledge.
Pretty sure they have a fair amount of mid-level casters? Stop your army outside of easy magic artillery range, send in your airborne or magic resistant skirmishers.
Keep casters in reserve to counter inbound magic hurlers.
Once the skirmishers are engaged with their artillery, either picking it off, distracting, or forcing it to use it's attacks less effectively, only then does the melee force start moving in, preferably with it's own low level artillery and counter magicians. Also, it's initial rush will likely be in skirmish formation, spread out to prevent extreme casualty from AoE effects.
Fireball and such will get through on both sides, but as long as the horde isn't just sticking it's face in the proverbial sink of razor blades, it's not a foregone conclusion. It becomes test of numbers versus quality versus tactics and strategy.
Remember, your villains live in this magic world. They will use tactics that minimize their casualty rates based on the most prevalent attacks in this world. They won't simply resort to real world tactics assuming a vacuum of knowledge of their own world.