Twilight Arbor is great. If guesting gets in any time soon I will happily do that one with you - I've done story mode on it twice and I sure wouldn't mind doing it again (doubly helps that I know it very well now). I'm actually waiting for guesting so I can start doing explorable as one of my friends who's on Desolation also really wants the Nightmare armor so we decided we're going to farm it together.
Also, I'd say that the Fire Golem boss in SE is worse than a Mallyx-type, as the dungeon leading up to it isn't terribly difficult at all (and actually quite funny), the boss directly preceding it is fine, and the final boss is supposed to be great...but the Fire Golem is just terrible. Think the Fire Elemental in BWE3, if you fought that at all.
(amusingly enough, I did that again recently. So much easier than before, but still hard. I wound up walking the group there through how to defeat it and we took it down in 15 minutes, with minimal deaths once I started guiding people as to what to do and where to go.)
I'm having the most trouble with Thief, myself. Which is weird because as how I play my Mesmer, you'd think I'd be used to such hyper-mobile combat...but I'm finding that my Thief feels way squishier than my Mesmer did at that level. Maybe once I level a bit more and have more pumped into my traits (I'm speccing toughness this time around, I think, breaking my habit of going all out glass cannon as I usually do), so I'll see if that helps...
Ranger is stupidly easy to play once you get a longbow, btw.