I found the normal Single and Double lines to be easy-medium difficulty. You will be facing very weak pokemon at first, but eventually you will battle partially-evolved pokemon and quite a few "mediocre" fully-evolved pokemon once you are halfway through the lines. Ingo and Emmet are the only challenges to the normal lines, really, but they were still possible to defeat as long as you had a pokemon with Earthquake/Earth Power. Emmet is slightly more difficult on the normal line because he starts off with a Durant, a pokemon with above average attack and speed stats, that knows Rock Slide.

The Super Lines, however, are filled with pokemon so powerful that I question their legitimacy in the games. Unless you have a pokemon/pokemons with very high IVs all around and are EV trained, I recommend not challenging the super lines. The pokemon you typically face on the super lines have remarkable IVs on top of their amazing base stats (because only the most powerful fully-evolved pokemon are fought in the super lines to begin with).

If you are on the Super Double Line, you will typically be paired up against pokemon that seem to work together to fit a strategy (i.e. Tyranitar and Cradily; Tyranitar's Sand Stream activates a permanent Sandstorm, Cradily's above-average Special Defense gets doubled).

Oh, and don't get me started on the legendary pokemon that you will begin to face starting around match 30. You will be facing up against a lot of Suicune and Lati@s.

The Battle Subway is difficult if you are not thoroughly prepared. As long as you give them a varied movepool and EV train them properly, you can get through the normal lines with the pokemon that you can catch in the wild. Once again, if you have pokemon with high IVs and are EV-trained, you should advance through the Battle Subway with very few problems. Don't be surprised if you do end up losing at some point on the super lines because the trainers keep getting progressively harder as you advance past each round.

I found the Multi lines (paired up with a CPU trainer) to be slightly easier than the Single/Double lines. The pokemon that the CPU are provided with have high IVs and great moves.