Wow...
What the heck are Nightwing made out of? O_o
Steel tissue. They don't need armour or weapons ha. So anything below steel would do next to nothing, and axes are the least useful things against anything armoured at the axes' own material or above. Candy armour, edged weapons and steel warhammers/maces are recommended against challenge races. You had no elite melee but you NEED them especially in dodging and with shields, plus
all armour coverage at the least with leather/clothing also full coverage underneath if you want to give them more chances to not die; I'd suggest danger rooms but it's not necessary. Also, force transference that Toady added in makes all combat more deadly to all combatants, so one direct punch from a larger creature made out of steel...Well it doesn't go well for the average military dorf. Multiple mail shirts and full clothing coverage underneath used to help immensely, I'd like to think with force transference they're even more useful? I wouldn't know about xbows, I haven't used them for much other than mass-hunting patrols and against the weaker races.
One Nightwing seems to have glitch-dodged through or over my wall, and ended up among my troops, and started tearing into my archers before I could do much about it.
Make sure your walls are two wide, it's been noted for the last few bunches of DF updates that any dodging can dodge through walls one tile thick, including diagonal.
This whole experience was baffling and terrifying, and leaves me utterly baffled as to how one could ever face such enemies in numbers without resorting to broken mechanics(bridges and the like) or cave-in traps...
The 10 Basic races and 4 Bonus races are more than enough challenge for most DF players, the Challenge races are for those desperate to face the most torturous ‼FUN‼ and painstakingly win against the worst of the worst. But most likely to perish eventually, it was inevitable. You're meant to think it was baffling and terrifying, they are built to
end fortresses that aren't supremely prepared single-handedly...success! However I'd consider Nightwings the second easiest of the four, but that doesn't mean they're 'easy' at all. If you get bored of the circus plus all the other races and/or you usually embark in Evil/Terrifying zones etc. and still want more
punishment...it's time for Challenge races. Or, as you mentioned, if you just want to exploit mechanics and laugh at them. It's up to the player and how they enjoy things, personally I only enable one or two challenge races at a time plus almost all the other races.