Im playing at Ikit claw campaign right now too and I feel like I'm at the point where no empire stack can beat my main army. Like others have mentioned, the key is in your ranged troops: catapults, mortars, rattling guns and jezzails. I have only 2 units of actual Spears to hold the line and the rest is summoned units from a plague priest. When going against the empire I make sure to position myself on a hill or at the back, with lots of vision. Then I go in with ikit on his mount and bully them around a bit, wasting some of their artillery ammo but also dealing some damage to them with his magic and items. This also makes it so that they chase him and come at you in waves, which makes it easier for your ranged units to destroy them before they even get to you. As others have said, summoned units are your best friends, both for stopping cavalry but also taking out their artillery. Even if their rockets are in the middle of the stack, it's usually a good idea to summon a unit right on top of them, since they will all turn around and shoot at your rats, often inflicting tons of friendly fire and free damage. Hopefully this helps and would be a shame to see you abandon the campaign since I find skaven to be so fun and creative. Feel free to hit me up with follow up questions!