Currently, AI Skaven players cast Menace from Below relatively early in the game, before melee combat has even started. Basically, as soon as the spell becomes available off the cool down, the AI is casting the spell.
On the plus side, the AI is now targeting the player's units as opposed to just adding the unit into its army and watching the unit degrade.
Unfortunately, the heavily outnumbered "Menace from Below" unit will likely be focused on and easily dispatched by the enemy since there is nothing else for the enemy army to do. Due to the fact that the enemy army probably doesn't have anything else to do if there isn't any artillery in range.
It would be better if the AI started casting these spells once the two players are in artillery range, i.e. once there is artillery fire or other missile fire, then start casting Menace from Below. Since there is other stuff going on, the Menace rats are less likely to be focused down immediately.
Other global spells like Doom Rockets should also be delayed for similar reasons … if the doom rocket or other global spell is used early, it is much easier for the player to move his units out of the blast radius compared to if combat is already underway.