I'm a lvl 67 Siren and have played most of the game through on my own but there are two missions which are impossible to do solo. 
 

• You will die- Kill Cramerax this guy kills me in two hits -_-
• 2nd time killing Knoxx and looting the vault. He has medics the second time and since it takes 1400 smg bullets to get him to half health it's impossible.

Well, first off, you should get to 69. Anyway, you'll be happy to hear that Crawmerax is apparently the easiest with Lilith. 
 
I haven't finished Crawmerax, but some things I found helpful (I am Roland BTW, so some of this might be off):
 

• Try to keep moving around.
• Pack a lot of health.
• Use the other enemies to get second winds.
  
• Make sure you aim at the glowing bits, I don't think you do any damage otherwise.
  
• Pack weapons with different status effects, as you will need them for the other enemies
• Respec your character if you need to, some skills are a lot better than others when it comes to Crawmerax
• Try to level up your proficiencies on the types of weapons you plan to use
• Using some sort of ammo regeneration class mod will help (or a regenerating weapon) You will need a lot of ammo.
• Don't worry too much about dying and losing money; deal with it :P