Guild Wars : Is it possible to finish the Underworld quests with heroes?
It is certainly an extremely difficult challenge, but it is possible to do it with one human player and 7 heroes, and here's a guide on how to do it: http://www.guildwarsguru.com/forum/7h-underworld-guide-t10480563.html
There are a few specific spots where having additional people would greatly help as heros can get overwhelmed quite easily. Also, having a few very specialized builds would greatly help. Namely, a obsidian flesh elementalist and a spellbreaker monk.
- The Four Horsemen - the enemies comes as two split groups here, most teams send 1 obsidian flesh tank + SB monk handle one side while the team takes out the other half.
- Escort of Souls - It helps a lot if one person triggers the quest while the rest of the party remains at the spawning point to kill enemies and clear the path. Otherwise, the souls will die.
- Servants of Grenth - same thing as the Four Horsemen, two groups will spawn, one towards the king one towards the reaper and you must protect both.
There are a few other tips and tricks that a skilled UW quester should know, I haven't done this in a while and the last time I did the mission was with my guild. It is highly recommended you go in with at least one person who knows what they are doing. Even a proper UW run with a prepared team will take ~1.5 hours.
Short answer, the more knowledgeable human players you can get, the easier UW will be. It will be extremely hard to do with just heroes.