Here's a tip I discovered while doing Frost Gate, which also helps for Dunes of Despair, when you are using Heroes & Henchmen.
Frost Gate: I equipped my ranger with Necrotic Transversal in order to do the "lower path" that Rurik travels. At (I think) the first bridge that crosses the path, I noticed that one of Rurik's escorts had died, so I used NT to travel down there. I flagged my H&H up top so they wouldn't get into trouble.
I wasn't aware, however, that there were actually enemies at the southern end of the lower path, so while I was checking the map, I got waylaid by a Mob and died. Normally I would have just said some expletive and restarted the mission, but for fun, I decided to un-flag my H&H and see if they could get close enough to rez me. (see Note)
When I un-flagged my H&H, for some reason, they began to run back towards the start of the mission. So, I figured I would have to flag them all the way back to me. When I went to put a flag back by the bridge where they first were, I accidently put the flag on the lower path. To my surprise, the H&H "teleported" to the lower path and I was able to flag them to me and they then helped kill stuff to complete the mapping. (This may take several attempts to get it to work. see Dunes of Despair.)
Dunes of Despair: I again (as is normal) used Necrotic Transversal to teleport across the gap to get to the "bonus" area. And again, I died. This time, remembering Frost Gate, I flagged my guys across the gap.
It took a few tries of flagging and un-flagging, but eventually, I got all the H&H across the gap and the full group cleared the rest of the map.
Use this tip soon before ANet "fixes" it.
Note - for those who aren't aware of it. When you're dead, you can click on other live members of the Party to view things from their perspective. This makes "flagging" a lot simpler when the H&H are a long distance away.
Note 2 - I was playing solo with Heroes & Henchies. I don't know if this works with other group formats.
EDIT: I got my Tyrian 100% and I still had some areas on the Ring of Fire to do (and without the Arenas). As has been said, there's about 102% available.