Level 90 hunters can solo Sunwell Plateau without much effort.
Tips and tricks
Skip the first boss as it cannot be killed by one player. You can kill all the bosses even if you skip the first boss.
The second boss, Brutallus, is a tank and spank fight. Just try not to stay in front of him. Be prepared with potions, flasks if you think your damage is too low.
I was able to solo the raid with level 90 PVP gear, all blues and around 4 epic pieces. Epic PVP weapon.
The third boss, Felmyst. Just avoid the green poison clouds that are casted by the boss. Nothing will happen if you lose control over your character (when you fail and get caught by the cloud). Wait 10 seconds and you will regain the control of your character.
Alythess & Sacrolash. Kill the blue boss first.
M’uru / Entropius. Try to kite the mobs with disengage. Just nuke the bosses all the time. First will be the one from the middle. After you kill this one, the second and final boss will spawn.
Kil’jaeden is an easy fight. Nuke the boss all the time. Cast a “Spirit Mend” when the spell is up.
I’ve soloed this raid using a spirit beast as pet, Beast Master spec, also the “Mending” glyph. This was useful for Brutallus that does quite some damage.
Good luck in getting your legendary weapon and transmogrification gear. Some interesting hunter items here: a belt with a skull called “Vindicator’s Chain Girdle” (it can be exchanged for a tier item that drops from here), looks great if you are going for an undead look, another epic bow called “Golden Bow of Quel’Thalas”.