>>536510246
You can definitively hit 400, even as a solo player. Though you may want to make multiple characters to tackle different kinds of monsters (I did almost everything on crusader as an example, it being highly survivable with good enough damage). I wouldn't call it difficult either, mostly just a matter of time.

Skipping mvps that require a large party, thanatos, thors, bio3, you would still hit 400.
Keep in mind that MVPs might seem scary, but they get significantly easier to solo as you go along and build up your collection of tanky cards.

Finding a few dudes to team up with shouldn't be that hard either, especially for MVPs that are monster of the day.