Update for post patch 6.0: This strategy no longer works since pet ability cooldowns (Spirit Mend in this case) are now shared between pets. The good news is you can just skip Valithria if you want now.
Valithria Dreamwalker is usually the main stumbling block for pure DPS or DPS/tank classes in Icecrown Citadel solo progression. In order to beat this boss, you need to heal it to full health while dealing with adds.
Lucky for hunters, we have spirit beasts with Spirit Mend. A single spirit beast isn’t going to be enough with a 30 second cooldown on the Spirit Mend. You will need at least 4, preferably 5, so you can rotate them in succession. If you don’t have 5 unique spirit beasts yet, I would recommend taming a few copies of the spectral porcupines as they have a quick respawn and always seem to be up — Hutia, Degu, and Gumi. These are challenge tames which must be brought to 30% health before you can tame them.