sneaking past the gnoll fight to take the high-ground from the archers turned it from a hard fight to an easy one for me, but the trip of getting everyone over there without being seen and deciding when to strike was great....I like it, it's the sort of tactical realism and dramatic tension I like.
This would still be a good strategy even if high ground didn't provide advantage. You'd be getting up close with their ranged attackers without their frontline to protect them, and it's a good defensive position for when their frontline does come up at you. IF you knock their archers down ,you can use the terrain to prevent them from having line of sight to shoot you, which is far better than the disadvantage theyd have if they shot from low ground in the current situation.
The terrain and verticality in this game make for very interesting battles even if the high/low ground advantage wasn't a thing.
