Slowing down enemies is good, but most battles are decided in the early stages when Knights reach and destroy a couple of stacks. To counter this, I recommend the following:
1. Never fight on Haven territory.
2. Find at least 2 artifacts from the Death set (speed -1).
3. Be sure to learn tactics.
4. And, of course, have overwhelming numerical superiority.
With this setup, the Knights won't reach you, and you can take them out with skeletons and liches, as well as damage them with a Berserker with Blood Fury. Then there will be much fewer of them, and the damage will not be as great. The others will not pose much of a threat.
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