Well, if you don't have enough knights, you can always try to squeeze in some black knights in a human army (if you haven't killed Meshmere, you can make some). You most likely have persuasion, so you'll already be at +1 morale, so using them will simply negate it. Royal Griffins on the other hand will also add +1 morale, so you will still have the advantage of positive morale.
I personally did it with a mage on impossible with Paladins, Knights, Horsemen, Guardsmen and Archmagi (for additional protection of the weakest stack) and it worked well. However, I can see how the Royal Griffins can easily replace the Guardsmen because of their decent magic resistance (50%) which will greatly help against Gremlion's mass attacks.
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