Well, granted, the fiend was outnumbered and not that skilled, but that was brutal
One thing you quickly learn when playing legend is that "big bad stuff" simply rarely survives. Just like ancient Romans could take down Carthages elephants, so can adventurers take down big baddies. The CA system is really nice but it does break down when there is too much difference in CAs on the two sides - either by players having 4*3 = 12 CAs and the Monster only 3 - or by a player casting Enhance(INT) combined with Enhance(Dex) to gain 6 CAs.
It is very hard to make an opponent which single-handedly can take on the whole party. The approaches I have found of doing this are:
1) Give it so much armour that most CAs will be spent not hurting it. However, this is rarely a good approach as low-damage PCs become useless (which isn't fun) and also because many adversaries can still be brought down by a trip or disarm anyway -- unless of course it's a giant or something like that. An example made by my gf in the first adventure we had in Legend: She was foreseeing and when they in an old Officer's bunk found a blanket, she tied the ends together and put it in her pack - then when they encountered a massive stone-and-flesh wolf (merge by a magical accident) she shot her crossbow at it with the blanket ... effectively entangling it's head. So, even though it had 3 CA, 6 AP and awesome stats, it was quickly brought to the floor and hacked to pieces.
2) Give it special-rules that allow it do make tricky stuff - negating CAs in a more interesting and engaging manner than the "no, your attack doesn't penetrate"-approach above. Examples could include an over-grown displacer beast wreaking havoc in a low-lighted tunnel, a beholder shooting 8 spells a turn or a troll spawning small troll-spawns from hacked-off flesh every time it is damaged.
3) Make the encounter more of a environmental encounter - for instance the adventurers are attacked by the cave itself when they enter it, perhaps the monster they're fighting is truly extremely large and heavy enough to throw people to their feet when it moves... or simply one awesome swashbuckler holding a crooked stair by himself. In these cases the encounter becomes more of a practical-riddle than a true combat encounter, albeit with a lot of combat style checks anyway.
So, don't expect adversaries to last long without backup
Legend is a very deadly system.. just as real-life.
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Author of the Eberron for Legend/MRQ2 conversion:http://runequill.com/files/Eberron_Legend.pdf