Difficulty Assessments
As a raid lead, one question I sought the answer to early into my team's progression through the operations was, "what's a good order to do them in?" as well as how difficult each boss was. It is important to me to set benchmarks for my team. "X" number of weeks/months on a fight/operation. Each group is different, and different compositions make different fights easier or harder. There are far too many variables to attempt to factor in to provide a "true", this is how hard this is. Below is a very general breakdown of what order I recommend tackling the bosses, as well as how complex I think each individual encounter is. This is highly subjective, ask me tomorrow and I will probably roll my own numbers a bit differently.
At the Master/Nightmare Level of raiding, I highly recommend troubleshooting in this format if your team hits a wall/does not progress in a fight for more than 5-8 raid sessions.
1) Switch Specs - Have your team swap specs. At the nightmare level, most players are proficient in 2+ specializations. If the team is unable to progress, try bringing a different composition.
When recruiting for NiM content, it is far more important to me that a player can play multiple specs adequately vs one spec exceptionally.
If swapping specs up does not break any walls, skip that boss. Try getting a Lockout to the next boss, or move into a different operation.
2) Switch it up - I find the key to NiM raiding is keeping the team engaged and wanting to show up for raid. I have been on 4 progression teams that have made it to Styrak, hit a wall, progged it for months and just given up.
I think it's important for new groups to "gel" first before hitting any of the proggy content. The first 5 bosses of SnV are generally "easy" and will get good mixing time with a new team. Zorn and Toth can be a bit of a rocky start for players new to NiM content and can demoralize new players/raid teams quickly.
3) Step back and practice new specs, then go back at it - Maybe the answer isn't either of the above. Maybe your players just need time, a productive atmosphere and a healthy place to practice a different spec. Players often want to try something different, but they are cautious to go into a PUG setting and torture players they do not know. Use a few weeks of raid time to invest in your players to make them stronger in the classes you may see benefit in them playing.