I finished Underbog at 64(minus the black stalker because I could never perfect the 'mushroom trap' trick)
The trick to first boss is 2 shaman off heal (down rank heals) when you need it, while the other 3 dps, and mix between 5 dps and 3 dps and I had no problems killing him before the first mushroom exploded.
The second boss you want to try and have your "main" shaman tank him with your back against a wall facing away from your other shamans, try to maintain aggro using frostshock on your main, while using 2 to lightning and 2 to heal. Poison cleaning and nature resist totem help I think...
3rd Boss, super easy, heal when needed but he dies quicker then a bog lord. His pet is nothing