Heya. I'm pretty sure that I've had a victory on every Skirmish Hall map against an Unstoppable opponent, but I have not completed the "hidden" quest for doing such. In-game name is Frogspoison.
Skirmish - Victory on All Maps quest not completing
- Thread starter frogs.poison
- Start date
Not sure this is exactly what you are asking about but; there is only a "Win on all maps" Trophy. There isn't one for a specific Opponent level. Standard AI will give you the Trophy for winning on every map. But other AI doesn't do anything after that.
Are you sure you won on the original maps? It should be the first 21 maps. It worked for me a few weeks ago for celts.