My one run in with mobs from my open air chasm has been a caveswallowman trapping one of my initial seven outside while he was unloading the wagon. So we had an unarmored and unarmed dwarf with no military experience (Pretty sure he was a non-miner as well, but considering 5 out of my initial 7 were set to mining duty, I could be wrong...although the miners would've been too busy mining) vs an undead caveswallowman. The dwarf shattered'em within a matter of seconds and had no injuries.
Not sure if that was pure luck, but I haven't worried about the chasm since. The only issue I've had at all up to this point was a lone harpy that triggered a stone trap. (Limiting my use of traps to three 3x1 stacks early on, to be removed once I have a standing military. Normally I'd not use them at all, but I can't spare a single dwarf for even temporary military duty at the moment. Even with four to five miners working, I'll barely be able to get my farm up and running before immigrants arrive. Between carving my extremely long entrance, of which there was a magma pool right in the center of, scouting out the underground river, which I got very lucky on, digging out the first level of my tree farm, and digging out the high volume water transportation system...it's taken a while ;p)