Yea, just hard. The game gets harder and spawns more monsters the more people that are on, and Sheddi and I survived fine a lot of the time fighting in different areas. Some of the mobs are cheap, some are just plain ridiculous like Wyverns(dragons), and if you fight more than a few at once you aren't going to survive like you can against 20 zombies.
You can do everything in hard core by yourself, its just hard core lol. Except for the boss fights. The boss fights are not possible by yourself unless you have full hallowed gear maybe, but to get that you need to have beaten all of the bosses several times each anyway.