Originally posted by Neyu:

Idk for the whole game experience because you still have to kill the bosses and Idk if it's possible solo (never tried)

However:
if you start with someone, do it with someone who just bought the game like you. Or someone who roughly has the same progress than you do it's more fun this way (imo)

I've killed all the bosses solo- several times before H&H and once afterwards.