I managed to duo the Crystal Lich on medium with a friend. (We were both paladins) However, I can't seems to make it work on hard, especially soloing.
But I may have figured out a none cheesy strategy. (A cheesy one would be that a paladin solos until boss and then a friend connects with a ranger to kill it)
Shared Hp pool is actually really helpful in my opinion. If you are 4 players with at least 2 of those being paladins, people can somewhat survive the falling spikes. If Paladins keep upgrading Hp and heal ability while everyone carries a Rejuvenation Potion then everyone should survive. With a group sharing around 500hp, one can survive almost anything, and just one potion can fill that entire hp bar. The paladin group heal can heal all at once, giving it a quadropled effect in the shared Hp pool.
My only problem with this strat is: I need good people to play with :3