Although you can actually jump griffins over a wall—let everyone hit them: then they will retaliate against everyone, the hero will deal extra damage and resurrect them (if they have the skill; mine usually does) along with the agnels. You sacrifice the griffins to beat down the entire enemy army:)
And there's no need to bash necs. That' same-ability is just brutal; they lower morale so much (and you can still follow up with a banshee scream), that enemies won't even be able to move; plus, you have to consider that taking leadership isn't worth it, and therefore morale will be lowered to the max.