I like Jelier's idea that Gwaihir can't Fly. I was going to suggest that you deploy as for the DOS Rescue scenario: Gwaihir in the centre of the board, the Evil force (see below) within 12 inches of him and Gandalf touching a table edge - The goal would be for the Evil force to kill Gwaihir within a turn limit.
I would suggest starting with about 15 Mordor orcs (not allowed to Piercing Strike, Feint or spear support) and say the game lasts 10 turns. Play it through and if it's too easy for Gandalf and Gwaihir then increase the number of orcs. Keep playing it through until the odds of winning are about 50/50.
I would also say that due to being wounded and enraged Gwaihir never counts as being trapped as otherwise I think it might be a bit too easy to kill him in one lucky round of combat.
The cool thing about this kind of scenario with so few models is that it won't take too long to play through, edit the parameters a little and swap sides and play again.
Above all have fun, it's what the narrative scenarios are about and this sounds great, in fact, I might even give it a go myself!
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