[Samhain]

One thing about AI, is it tends to not feed those big armies very well. Starvation really brings down morale, making those routes especially effective.

You can also kill them with your own armies. But, compensate for your fewer numbers with more mages using area effect spells like Magma Eruption, Iron Blizzard, Shadow Blast, Blade Wind, Falling Fires, etc. Fatigue will likely be your biggest issue here, so look for means to mitigate it, like gems, magic path boosters, or an Earth Bless for sacred mages.

Buff your troops with Legions of Steel, Mass Regen, Arrow Fend etc. to keep them alive but set most of them to guard your mages. You may need of light screen of expendables to take a cavalry charge. But, otherwise count on any who march forward against those odds to be lost. Quagmire is also useful to keep the enemy at bay longer to give your mages more time to do their work.