well...

he can do quite a bit
if this is level 11, he has 89 base AD + 55 duskblade + 60 stormrazor + up to 20 from shards + (variable by runes) and then i'm not sure if he can cast his Q before auto attacking but if he can, that's 60 more AD
284+ total AD , 224 before Q
ult deals 275 + 120% bonus AD ; if we factor in that he might not get to Q after, he deals 414 from his ult, 100 from duskblade , first auto attack then deals 1.4 * 224 = 313.6 + 20% AD from passive = 44.8 , Q then deals 245 + 75% bonus AD = 349.25 and then adds 60 AD to everything after , E deals at max 170 (don't know build order) if not, 80
it is then a variable for what occurs by what the user chose for their build and if the opponent gets away or not and if the nocturne has ignite or not
275 + 166(from AD) + 100 from duskblade + 358.4 from first auto attack + 349.25 Q + 170 E, we get 1248 physical damage with 21 lethality minimum, which at level 11 reduces their armor by 17.7 , 170 magic damage max; then variables
if nothing procs, lets take lucian as the example since he's the current meta
lucian at level 11 will have 54.3 armor - 17.7 = 36.6 , magic resist 34.4
100/136.6 = 73% damage taken from physical ; 74% magic damage taken
with these numbers, with no external influence at all, nocturne deals 911 physical 125 magic damage from lucian's 1325.7 hp
with runes and variables taken into account, these numbers can get much higher, within a time span of about 0.7 seconds
#ignite alone gives it enough to be a kill at 330 true damage
lucian can adjust this a bit by opting for armor shards, but the result is almost the same
100/146.6 = 68% taken 848 physical , 125 magic , incidentally, if the person's runes are terrible, this gives him enough to survive ignite, but the odds of making a non-offensive setup lately in runes is fairly low. The odds of not getting a second auto attack is also low, and nocturne can also apply jungle damage buffs to his auto attacks
#this is also taking into account the worst possible situation, so who knows. I don't even know if it's possible to have those items in nocturne's build at 11, the tally being 5700 gold