Normally the best way to beat a late-game-teamfight comp is to get a crushing lead early and not let it reach late game.
I was thinking of guerrilla warfare but this usually ends up with a normal teamfight.
You can try to pick someone off and then force a 5v4 that you can (hopefully) win. They have to get sloppy, though.
Splitpushing is a no
Asymmetrical warfare would be another good option. Not sure why you've ruled it out.
If you have a much better splitpusher/duelist than they do, that player should be able to take towers faster than their team can push to your nexus 5v4 if you stall them out through poke/disengage/waveclear. If one of their players isn't enough to deal with the split push, you'll probably win (or at least you have a chance). Or they'll have to send two players to deal with the split push and then you can force a 4v3 fight or take objectives if they fall back instead of fighting 3v4.