From my experience with Ascalonian Catacombs it was relatively easy to complete the story mode with minimal preparation. Most of the difficulty is knowing which enemy to prioritize in a group. (hint: the answer is kill the elementalists first)
During a boss fight if an ally goes down then get try to help them up before they're defeated. If they're defeated however then they should head back to a waypoint to get back into the fight as soon as they can so as to keep damage up on the boss.
During the necromancer boss fight make sure to kill her minions quickly when she summons them. For the bossfight against Ralena and Vassar make sure to kill Ralena first.
Other things to watch out for:
-Mesmers. They reflect ranged attacks when they put their shield up so it is best to kill them with melee attacks.
-Monks heal other enemies that are with them, however don't focus on them when there is an elementalist around, since elementalists deal too much damage not to be killed first.
-At the beginning of the dungeon you need to open crypts in order to get the door to open, make sure to open them one at a time and kill the ghost that spawns before opening the next one or you will be swarmed with ghosts and probably die as a result.
Overall the dungeon isn't too difficult, though it is harder than the next dungeon. The explorable mode is much more difficult, though more in execution than actually knowing what to do. (It probably didn't help that the group I tried explorable with chose the hardest route)
