The Shandal Empire wasn't in a rush to end the war. It was the opposite.
Being against three of the most powerful organizations in those Mortal Lands made the outcome of the war obvious. The Empire couldn't aim for victory without the help of their Patriarch. So, it did its best to appear difficult to conquest and defeat.
There were only two possible outcomes to that war: A complete defeat or a surrender with conditions. Being difficult to conquer would improve the terms of its submission.
The more the invading forces suffered to advance, the more generous their offer to the Empire's surrender would be.
Of course, the invading forces wished for the exact opposite to happen. They knew that destroying the Empire was almost impossible since they were fighting it in its home. Still, showing their superiority on the battlefield could force it to accept harsh conditions.