Walkthroughs for custom maps.

I wrote on the forum before that you should only level up and train armorers for these garrisons: Tazarr (better) or Mephala. Tazarr must also have tactics and attack (this is a must-have in an anti-magic garrison).
And you can only break through a garrison with 9999 defense by using skeletons farmed throughout the entire map (I had about 180,000, but you can have more). These skeletons, especially when using the terrain, can break through, particularly when they are in a prayer formation. Also, take a bone dragon and an archdevil to the garrison to lower the garrison's morale by -2, which will also help a lot.
I don't know any other way to break through a garrison with 9999 defense; accumulating strength for 12-20 months or more is a bad option.
Yes, I forgot to add that in the 9999 garrison, there is a "stash" where you can hide your main stack so that it is hit by only 1-2 enemy stacks, no more, which will save on losses.