I may have posted one at the time, but none in right now. One I recall had 3 or 4 hills starting 2 tiles over from the beach head. I actually marched a bunch of armies to attack it, rather than land near it.
I block a lot of tile in a line from sea to sea, maybe 9 tiles. I then moved in and took the town. Then I positioned armies around the three sides of the town. I block two tiles deep on all three sides and opened a large hole so they would move to the hills.
I opened a complete path to the town, but thye had to circle around from south to north passing to the east (hills). After a large number, iiirc about 700 units, go to the hills, I closed the hole by the town.
This time I actually killed all the units over time. Once I chopped them down to maybe 300, I open a line so they could move to the town, passing between armies to take ZoC shoots.
These were all infantry, so it took then several turns to get there and they would send damaged units back to the hills and would pick those off.
The 900 stack was in an area that had been towns of theirs and they just happen to keep sending units, but doing nothing and I finally realize I could trap them all. The first trap was cover a large area I think. I eventually got them to move to one tile and I needed only 9 armies.
I had run into that before on defense, where I could trap groups from time to time, while keeping large numbers out of the fray, till I could either kill them or the towns.
It is not something I am able to pull off all the time. You need the right game, right numbers and for me it only happen when I am going for conquest. I need about 25 armies or more to do it.
Next time I see one developing I will try to remember to shoot some screens. I have been playing out AoI I and II and the Great War scenarios for weeks, so not getting standard games.