Before patches 1.5 and 2.1, I never trained knights – only archers. But Necropolis also amassed hordes of skeletons. Back then, these were the two most invincible races in a long game (IMO).
And now, 15-20 additional knights per week is also very good in a long game. These are level 6 units. And they are better than the Necromancer's counterparts (IMO).
Goblin shamans are very effective against the AI. Sometimes, because of them, the AI might completely stop using magic and only attack with melee units throughout the battle.