5-1: This mission is quite a handful, but with a decent party you can win no problem. The best bet, or your best bet, is to find a MNK who is willing to 100-Fist the main mob. He'll spawn little skeletons that have less HP, but it's better to just ignore them. Find a healer or two, a MNK, and have at it. Obviously, a MNK is not needed for this fight. With a regular PT you'll have to be careful, and probably lulluby any of the skeletons that come out, or have a couple of people ready to take them out as they come.
5-2: 5-2 is actually significantly easier than 5-1, in my opinion. After trudging (and I mean trudging) up through the Castle, you'll hit the throne room. It's set up like a big BC circle, with stairs at the very north end that lead up. Here you'll face the Shadow Lord. His first form isn't too difficult, with only damage during his Invincible to worry about (so bring a mage or two). Once you beat his first form you'll get a cutscene and then the Shadow Lord will charge your group. You're going to want to have the low levels and mages run north, to the stairs, while the rest of the melee fight him where you entered the BC. He has a nasty AOE move that he spams that can kill low levels. This form has significantly less defense and goes down pretty quick.
As for help, I've always wanted to try and solo the Shadow Lord. So let me know when you're getting a group together. Best of luck!