We defeated the bosses in Elephant Graveyard pretty handily (Rachel and I) and weren't really having much trouble at all. There were a few close battles where health got low, but those were more due to inopportune fizzles than anything.
We get the quest to go to Mirror Lake, which is accessed right from the Crossroads near where you start Zafaria. In we go. Right away, we find ourselves facing Tse Tse Snaketail. We load up a couple of storm shields and make short work of him and his death minion. He does have a rather high health total (7000) for a storm guy.
Next, we listen to Morganthe blather about this or that, then she summons four spectral guardians to fight us. This fight took a little while, and I wasn't familiar with the cheat mechanic. As far as cheat mechanics go, it was rather easy to deal with. I told rachel it would be a good idea to load up on treasure card blades for the fight, and then we started it up. What I initially assumed was that at any time the spectral guardian boss (Elephant Spectral Guardian) might catch you flat-footed without a blade and hit you with all the cheats (smokescreens and blizzards). What really happens is he randomly targets one player, puts a mark on them, and that player needs to drop a blade when marked. That player has to do it at their next opportunity. The other cheat he has is his super tower shield, which operates like every boss with a super tower shield. If you damage him, he interrupts and replaces it, and if he's the last creature up, he stops putting the shield up. Spells that remove shields without damage don't trigger the interrupt to replace them. So all we did is AOE damage spells until he was last guardian standing. The myth guy will cast earthquake and remove your blades and stuff! Just saying.
There were two more fights after this, each with four identical monsters. These guys didn't even cheat! The first group was 4 Goliath Bruisers (big bugs, ice, just under 8000 health each), and the second group was 4 Starburst Spiders (storm, around 6500 health). Easy! Right? This is where Rachel and I stumbled. Both groups hit hard and hit often. Figure on taking on average 500 damage every round from each of the Goliath Beetles. We each died and had to run back into the instance (luckily, not dying at the same time). Then the Starburst Spiders. There was a lot of dying. They score critical hits often, and one time we both died near same time and had to start the whole thing over. We took advantage of this, refilled our potions, threw in a couple more heals and shields and went again. It was ugly, but we limped through this. With a party of four and a dedicated healer, this wouldn't have been nearly so bad, but be warned! Lots of damage. Not quite sure how this might be soloed.
Katherine Light offered some words of advice here: http://www.wizard101central.com/forums/showthread.php?t=226370
She mentions picking up some resist gear and building your hits with multiple blades and traps before firing off. She also suggests that players save their attacks to go off on the same turn, so that one person doesn't become the target of all four monsters (which happened often between Rachel and I). Also, don't forget that these guys have stun cards (notably, Frost Giant and Storm Lord). Also of note, after these battles, lots of different kinds of treasure rewards :)