Let us take a simple example - Xorian Cipher. Did you ever notice that archers have an annoying tendency to jump backwards every couple of seconds you're right next to them? Did you know they also do not follow you (or contribute to dungeon alert) in a quest? If you use this simple annoyance you can have the ones in the right-side of Xorian Cipher stand on the platforms. Then you and a hireling stand on the other two. A "four-man" quest has just become one man and a hireling.
As for actual difficulty, it only truly depends on your skill as a player. Build and gear certainly help at times but they aren't the main factor in a niche game like DDO.