This scenario is added to the Mists of Pandaria as of Patch 5.1 and focusses on the Assualt on Zan'vess as the name implies.
The scenario is made up of several stages that take place as you make your assualt. The scenario even allows you to fly a chopper and launch rockets on several towers that keep a shield in place above Zan'vess. While gimicky, it works, and makes the scenario a lot of fun and something a little different.
Stage 1: Join the Assault
In this first stage you simply need to jump onto one of the gyrocopters and join the assault.
Stage 2: Defenses of Zan'vess
The second stage has you flying around the island and blowing up the various shield generators so that you can land and make your assault. The first towers are easy, just shoot them, the second set requires you to take out a group of mantid that are shielding the towers. The last group are being moved by some giant scorpids, and they must be taken out before you can hurt the towers.
Stage 3: The Heart of Zan'vess
In this stage you need to defeat the squad leader Bosh and then make your way to the Heart of Zan'vess.
Stage 4: Weapons of Zan'vess
This is the final stage and requires you to defeat the Commander Tel'vrak, which is no easy feat since a lot of adds will be summoned during this phase. Make you you call in air strikes whenever there are a group of adds as it will make your life a lot easier.
Assault on Zan'vess Scenario Achievements
There are only 2 different achievements that can be earned in this scenario.
Assault on Zan'vess Complete the Assault on Zan'vess scenario.
For the Swarm Defeat Commander Tel'vrak without killing any Zan'thik Swarmers.
This is a difficult achievement to get since the swarmers will stack up quickly and can cause a lot of damage. Make sure everyone has full raid gear and save your cooldowns for this stage and it shouldn't be too hard. If you have players that are not pulling raid level dps (40k+ DPS as a bare minumum) then it might be better to wait a bit to try for this achievement.
Comments and other ways to earn the achievements
Listed above are our tips on earning the achievements in this scenario, there are many other ways to do them as well. Why don't you share your methods in our comment section below.