I just started the Companions faction with a new character. I've been given the Radiant Quest "Trouble in Skyrim" which requires me to clear out the Falmer in Shimmermist Cave.

It's been very difficult, but I've gotten to the bottom of Shimmermist. I play on Expert, my character has just reached level 7. The final room of this dungeon has a Steam Centurion and a Falmer Gloomlurker. According to http://www.uesp.net/wiki/Skyrim:Falmer, Gloomlurkers are level 38. This guy kills me in 1-hit with most of his attacks, it seems nigh impossible to defeat him.

Don't get me wrong, I love the idea that there is an area of the game that I will have to come back to later, once I'm stronger. But it seems I cannot progress through the Companions quest line without finishing this quest. Is there a way around this without cheating or abandoning the Companions for 10+ levels?

Reload to a save before you got the quest. Then talk to whichever quest giver again, see if the quest you are sent on changes to something a bit more realistic for your level.

lower the difficulity for the room and then set it back to expert.

use sneak and a bow on both of them. stay away from the Centurion's steam breath.

if i remember right they don't like fire or sparks.

its likely a level 7 gloomlurker, which still might be quiet challenging but not impossible depending on how much you want to suffer.

I was given the same radient quest at level 10ish. That Falmer beat my ass at that level in melee combat. I came back at level 15 and had my indoctrination to the glorious way of sneak/archery. It took me a half an hour or so to ping him down (I had never done the snipe & dash method before, and I ran away much farther than was neccessary I now realize). But good lordy, it was a good feeling when all was said and done. The centurion can be easily killed if you keep your distance and shoot; he can't fit through the tunnels leading up to the room he's found in.