Naxxramas (10 man) is definitely soloable at level 90. None of the bosses require “very special” strategies except tank and spank. A fight usually lasts from 2 to 5 minutes.
Spec: beastmaster. Pet: Spirit beast. Gear: full epic PVP and a good PVE trinket. Half of the gear is upgraded 2/2.
Talents: Exhilaration, Dire Beast, A murder of crows and Barrage.
You can skip mobs by using Camouflage.
Some bosses that require a little more than tank and spank.
Thaddius. Try to kill the two mobs at the same time, at a few seconds distance. After that, jump to the platform to fight the boss.
Instructor Razuvious. Nuke the boss, ignore the two adds.
The Four Horsemen. This fight will hurt the most. If you’re not after the achievement, kill each of the four bosses one by one. They do a bit of damage but you should be fine.
Kel’Thuzad. This is the hardest fight and I died once. You need to full nuke the boss when he is up. Burst with all the damage. Use a corehound for heroism when the boss is about to enter the fight. Then, use: Bestial Wrath, Stampede, A murder of crows, Dire beast, trinkets, Rapid fire etc.
Kel’Thuzad has a spell that will freeze your character and you can’t do anything. You will die if you get frozen within a few seconds. But with enough damage, you will kill the boss before you get frozen. Is not very hard, you should definitely try it.
Rewards? I got a nice bow, Accursed Bow of the Elite. And also a companion pet that I’m now trying to sell in Auction house.
Most of the achievements are easy to do.