The best tip is to ignore the timer. Don't pay too much attention to it to cause yourself unnecessary stress. There is plenty of time, we completed this mission last night on our 4th try with 13 mins to spare. A party of 2 human warriors and the rest henchies. We didn't do the bonus, failed to work out what that was (probably return for that at a later date).
Our first couple of tries was to run it quickly. Go in, grab the crystal and get out as fast as we can. That didn't work so for the following tries, we spent a little longer with our movements. Where to go, which mob to avoid and kill those who may pose as a threat. Between the two encampments, there is a hill with a path behind it. Make use of it as there aren't foes there.
As you come down from the shrine (where the ghostly hero stands to recieve the crystal), we took the left gate and left camp first. Kill any roaming patrols to give more free space. NOTE: After grabing the crystal, there will be some respawn in this central open area so taking afew groups out first does help later.
We didn't use the bridge as it was too heavily guarded. There is another way in to the right of the bridge. Pull a couple of mobs out from here and kill them. They're in groups of three or four and not that tough. Kill the sages first as they're good healers. Getting to the crystal isn't too hard if you keep away from any mobs and wait if you have to so they don't end up in your aggro circle.
After returning this crystal, go back towards this first camp and to the right of the bridge. Instead of going into camp, go right behind the hill and this path will lead you to the other camp. Using the nearest entrance, kill those in your way and watch your aggro circle. Pull mobs out if needed to avoid aggro-ing more. Once you get the crystal, return the way you came, that is, behind the hill. If you have kill afew of the mobs, in the central open space at the begining, then there will be plenty of room to run through the mobs.
I've seen people forming parties looking for players who has speed skills. You don't need to run it as there is plenty of time to do the mission.
Good luck