They may have fixed the infinite focus bug with the last patch. Regardless you should be able to kill anything in 1 thousand cuts with the right setup so it's irrelevant. I consider this to be cheese personally so I'm not an expert on it, but:
1. Make sure you are using the tier 3 version of the ability with 300 focus.
2. I think the most important way to increase the damage is with the fully upgraded Mighty Offense Tonic since it gives +100% crit damage. You will want a high crit rate to make as many hits as possible criticals with this effect active.
3. Might as well use tears of the dead as well, not sure what else you could do with the 2nd potion slot that would add more damage (ooze grenade with 50% debilitate maybe?)
4. I imagine that the 50% damage bonus from Stealth would apply to thousand cuts just like anything else so might as well cast it out of stealth along with the Subterfuge passive that sunders when attacking from stealth.
5. To really maximize it bring Cole and use mark of death before the thousand cuts, then make sure to manually trigger it afterwards. This should at least double the damage, maybe more.
Yep. To pull off the solo one-shot, you need a high crit chance and Mighty Offense, and ideally another potion (doesn't matter what kind). Mighty Offense for the big boost to crits, and the other potion because, for tempests, every potion consumed gives them a stacking +15% damage (for comparison, that's half of the maximum berserk buff!), so you want to consume multiple potions. The potion passive is on the right side of the tempest tree, IIRC, make sure you have it.
Plus, as with all rogues, you want sundering or armor penetration abilities since dragons are heavily armored.
Also, Sera probably has it if she's specced as an archer, but make sure she has Pincushion. Ten seconds of a stacking damage buff for every hit... yeah, combine that with tier 3 TC and it stacks up pretty high.