I think I can help you at least in story mode. First of all NOBODY should go there without endurance/ vitality/ healing power (or Watchtower). Either the gear should be towards that or the traits, but the balance between survival and damge is still unknown to me. Second, remove condition skills can be very helpful, as you may get killed by that.

And now I will speculate: I believe the reason those rangers before the rangers boss are so tough is because the game was not designed to have all of them hit the same person at the same time. Considering nobody is too squishy, I believe 2 people could keep 2 rangers (maybe the more tanky people) occupied while the other 3 kill one of them.

Hope this helps. I ran it 2 times and the first it was as frustrating as you say, the second was pretty fun.

Edit: all the bosses there except the twins are pretty much kitable (heck, it is supposed to be easy mode) and you can avoid the melle damage/ spread it around the group pretty easily. So I will assume you aren't having too much problem with them. I don't think you need a very optimal group for story mode also, the extra endurance / vitality makes a huge difference. If nobody is too squishy it should not be too traumatic.(osrs gold)

And buy the gear cheap from the AH, I just bought a whole set for just a couple of silver.