Deck Defender

First thing's first:

• 
  
• Be sure to tick the box to 'track' this achievement.
• 
  
• If at any point in the encounter, the achievement text to the right of your screen turns red, then you have failed the achievement. Jump over the edge of the boat to die, and then come back and try again!
• 
  

Now then, let's burn down some drakes. I repeat, let's burn down some DRAKES!

• 
  
• Kill the drakes, using whatever abilities you have.
• 
  
• As soon as any drake appears and is within range, kill it!
• 
  
• Do not wait until a drake drops its riders. Kill it IMMEDIATELY!
• 
  
• Kill the riders as a secondary priority, not a first priority.
• 
  
• Soak the puddles as you can (stand in the swirly things that occasionally appear).
• 
  
• When Blackhorn finally lands, engage him and burn him down.
• 
  

In case I haven't stressed this enough somehow by this point, KILL THE DRAKES~!

SOLO-GUIDE:

You need to kill the [npc=56855], preferably before it lands, to make sure it doesn't cast its spell [spell=107439] - thus the macro below is handy to quickly target them (as long as you have ranged spells). I believe there are 3 waves, with 2 of these drakes each time. I did it on Normal mode, which is probably recommended (though not required).

Preferred (though not required); some kind of ranged damage spell (I did it as a druid).
Macro;
/target Twilight Assault Drake
/cast <YOUR SPELL HERE>
(Go to spellbook and SHIFT-CLICK the spell while the macro-window is open, in my case simply "/cast Moonfire")

Simple and quick achievement, remember to track it (SHIFT-CLICK it in the Achievement tab) before starting the fight - if it changes color at any time during the fight, simply jump over the edge to die and reset.
Simply finish the boss to complete the achievement afterwards.

Good luck, have fun and happy hunting!

If you kill the drakes before they fly off, either when the adds are still riding them or just after they drop the adds off, they will never shoot Twilight Barrage, and you will get the achievement by default.
The only trick is that the adds have to land on the ship, whether you shoot the drake out from under them or wait for them to dismount. Otherwise, the encounter will bug out and you will die.