It works very well online, providing there is good communication between the heroes. I don't mean secret communication via PM, as would be the case in something like Diplomacy, but what I mean is that there needs to be a clear understanding of who is going to do what, and in what order, posted in the thread.
I think that the problem that you've encountered as a group is that there is some rather impulsive decision making taking place, rather than a discussion beforehand as to how best to approach a combat, and that this is what has put you in the difficult position in which you now find yourselves. Sort this out, and you'll start having more success.
Also, I'm not sure whether a leader has been nominated for the group. If there is a leader, he has the final say on who acts in what order if there is disagreement within the group, and can control absent players' characters too, or nominate someone else to control them. Having a leader for each quest may also help.