They are nasty at low levels. Some poison resist equipment or potions help. Otherwise just do what you can not to get surrounded. Stealth in carefully, and when you engage move backward so they don't draw you further in where their little beasties and more Falmer can swarm you.

Spellbreaker is your friend. It's Ward effect will absorb a Chaurus' poison spit just the same as a Dragon's Fire/Frost breath. Also works on the Poison Puke of the Afflicted.
My Dunmer Nightblade has absolutely no problem with Falmer or Chaurus, but that's because she has 100 Sneak with 5/5 Stealth and she's a Vampire to boot, so another +25% to Sneaking, +25% to her Illusion spells, plus she takes almost no damage from Poison (Even though Vampirism gives +100% Poison Resist, it's still capped at 85%).

I hate mandibles.

Yep. I learned the first time I got accosted by a group of 6 Falmer at once. Whoo boy. I don't know if it was an accident or it was the AI but they surrounded me and I was beaten to a bloody pulp. Sneak is crucial, then it's better to draw them to you by alerting one you see in the distance then fall back (since there will be no creatures behind you). That way you ensure that you aren't rushing in and alerting what may be 4+ Falmer congregating in the same proximity. If you're a mage, runes work great with this method.

The falmer are harder than bandits because Bethesda didn't create every creature in the game with exactly the same strength....

Yeah they are really tough, my characters almost maxed out with really good armour and weapons but I entered a bandit lair and a group of falmer shadowmasters quickly sorrounded me, I used the become ethereal shout to get to a staircase where I could fight them individualy and killed them a lot easier but a few things for lower level characters would be sneak attacks whenever possible, poison resistance, using shouts like unrelenting force and ice form etc to not get overwhelmed, raise zombie spells or summons to divert attention from yourself, try seperating individual enemies from a group and if they're still to tough then simply turn the difficulty down.

If you're going to go toe to toe with chaurus, you will probably need to move the difficulty down. I find neither they or the falmer are a problem to my higher level sneak archer with the 3x perk.

That makes me feel better. I went in there at level 6.

Poison resistance is key to stress-free exploring in falmer/chaurus-infested underground lairs. Keep your eye out for a necklace of poison immunity in shops, which completely eliminates the problem if you're lucky enough to find it. One Windhelm NPC who ends up dead if you complete a certain quest may have a necklace of poison resistance (50%) you can loot off of him.

I usually go Ilusion/conjuration/two handed/destruction/dual wielding/one handed/archery/with a shield when fighting them.

Oh lord I know your pain. I had them as my first quest for my unarmed bad ass build. It took forever but I did it. Just need to manage a way of slowly dividing them up. They are strong, and adding the numbers they are usually found in makes it far worse!

I usually just nuke 'em from orbit. It's the only way to be sure.

I was playing my level 34 Redguard last night and ran into a group of them and I struggled. I even ended up losing a follower who was also armed to the teeth....buuut I may have nailed him with my Marked for Death shout. My mistake.

I did fill some soul gems, though.

I learned to love Chaurus Reapers after i happened to run into one at lvl 12 yikes.

If you're in a den where there are multiple charus', if you can, get on a ledge where you can duck behind a wall, then peek out and hit every one of them with Rage. Keep doing it until there is only one left standing.

According to the Bethesda Official Skyrim Guidebook, ALL of the quests are leveled, some higher than 30. The Guide also tells you the recommended minimum level for each cave, fort, etc.

I always start off with muffle (the spell cuz my boots are unechanted) as soon as I enter into a falmer area. Sneak attacks usually bring them down in 1 hit or low enough for a quick kill. With charus and reapers I usually just charge in like a mad man, swing like crazy, and chug health potions.

I should add that I too had difficulty with them initially, but once I learned they couldn't see sneak became king.

I always use sneak and I take sneak training from the Khajiit every time I can so my sneak rating is always very high.
While the Falmure may be blind they have super hearing and sneak not only lowers your profile but it also makes you more quite.
You have to admit, for blind people they're really good at arachery.
As far the boots, I prefer the "Boots of Brawn" so I can carry more weight.

They are one of the main reasons I like Redguards and Bosmer (for their poison resistance). That and Savior's Hide can seal the deal with poison resistance. I like bows too - I don't like getting up close and personal with chaurus or spiders .

Shimmermist Cave was where I learned the art of shoot-run-hide. My very first character, a Khajiit archer, got sent there by the Companions at level 7 or so. Uthgerd and I got rocked, so I left and came back at level 15 (with no follower). It was still extremely difficult, and the only way I could deal with the chaurus was to ping them with arrows, one at time, then hustle back to a turn in the tunnel and hide until they didn't detect me anymore. Rinse and repeat for the Falmer and Dwarven Centurion. I think it took over 90 minutes to clear that dungeon. I haven't set foot back in there since, more than 6 characters later!

One of my favorite things to do with Falmer and Chaurus is to take my necromancer and make zombies out of the corpses, then set them on the remaining living ones. It's very satisfying!

Just an FYI-

Detect life sees them when they are hiding in their holes in the walls. Slap a rune on the hole, then they take damage and you get a free warning when they pop out.