- What Gym Leaders are an issue? This means any tough or even simply annoying ones.
Probably not a problem for most, but the fact that his Lillipup is level 13 and your Pokemon is probably about level 10 is scary in a Nuzlocke. Grinding is very annoying as well, as the highest level Pokemon you can fight are level 7.
Elesa is considered one of the most infuriating gym leaders to ever exist. She likes to pivot with Volt Switch, making it very hard to get in a super-effective hit. Until you take out Emolga, she has a free switch-in for Ground moves. Zebstrika has Flame Charge to counter any Grass types you may send out to resist Electric attacks. Flaffy pivots in and out, spreading Paralysis.
Drayden can be a hard fight if you're not prepared. His Dragon types are all quite strong. However, he doesn't have many Ice counters; mainly Rock Slide on Flygon and Revenge on Druddigon.
I'm mainly including Marlon because of that terrifying Shell Smash Carracosta. Luckily, if you can outspeed it, you'll probably one-shot it with a Grass or Electric move.
- Which Pokemon should I try to get if I can? For example, is the Zorua in Driftveil City worth obtaining, levelling, and evolving? Are the Musketeers (Cobalion, Virizion, Terrakion) worth the potential risk of them KOing me easily? Is Volcarona worth the detour?
Krookodile- STABs are Super-effective against half the Elite 4 and overall useful. You can find it as a Sandile on Route 4, the Relic Castle, and the Desert Resort.
Magnezone - Magnezone can handle Marlon and Skyla easily. It's Steel typing is quite useful in-game. You can find them as a Magnemite in the Virbank Complex. Level it up (past level 30) In Chargestone cave to get a Magnezone.
Galvantula - Another highly rated Electric type, this one can be found as a Joltik in Chargestone cave.
Lucario - (What's up with the Steel and Electric types?) Lucario is a friendship evolution, meaning you need to have it in your party a lot while you're walking around. It's Fighting type comes in handy many times throughout the game, notably against Cheren, Grimsley, and Ghetsis's dreaded Hydreigon. It can be found as a Riolu in Floccesy Ranch.
Swanna - Useful HM slave. Can be found as a Ducklett on Driftveil bridge. Good for Clay, which is right there in Driftveil city!
As for Zoroark, yes, it is definitely a great Pokemon to obtain. It learns good moves and has nice stats for in-game. Besides, think about how much fun it would be to troll Caitlin by disguising it as a Fighting type! In all seriousness, though, Zoroark's STABs are super-effective against half the Elite 4, random trainers, Marlon's Jellicent, etc.
The Musketeers more difficult to decide upon. Obviously, you wouldn't put a Cobalion on your team if you already have a Lucario, right? It just depends on what your team needs at that point. Virizion is definitely a great one though, as it helps with Marlon, Grimsley, Neo Team Plasma, and Ghetsis.
Volcarona is definitely worth the detour in my opinion. It learns Flamethrower by TM (found on victory road). I taught Signal Beam and Giga Drain to mine. Absolute powerhouse.
It is a little useless until you can get those moves though.
- What's a good level to be at for each Gym and the Elite 4? I'd like to be levelled so I don't steamroll everything, and opponents still pose a challenge.
Cheren - Level 13
Roxie - Level 19
Burgh - Level 25
Elesa - Level 31
Clay - Level 35
Skyla - Level 40
Drayden - Level 50
Marlon - Level 53
Mid/low-60's across the board
Again, low/mid 60's
- Are there any "trainers of death"? For example, in X and Y, there is a Double Battle with two Furfrou that often poses immense trouble to people fighting them. Are there any trainers like this to look out for, and how to beat them with little hassle if so?
Triple battle trainers are immensely annoying, as well as those with high-level Pokemon. I'd watch out for the Triple Battle trainers mentioned here. One trainer that gave me some trouble was the Veteran right before you enter the cave before the league and fight Hugh.
- Any other useful advice that may help. I'm not a newcomer to Nuzlockes, so I don't need basic suggestions.
I don't have much advice, other than look up the teams of important battles beforehand. You probably already know that though.
Gym Leaders and Elite 4
Pokemon Black 2 Routes
Good In-Game team for BW2
Team Plasma Bulbapedia
And of course, some experience :P