Relevant Talents used:
Body and Soul
From Darkness, Comes Light
Patchwerk has next to no mechanics, and yet it is one of the more challenging bosses in WoTLK. When soloing, especially as a caster class without any permanent pet tanks, this boss is impossible prior to patch 5.0.4. However after the patch, healing has been buffed significantly, for all healing classes, and therefore shadow priests can now have a shot at this boss.
The boss doesn't hit that hard, but it hits insanely fast. That is due to the addition of Hateful Strikes which were not supposed to land on the MT, but OT instead. However when soloing, the MT takes both the normal melee hits AND the Hateful Strikes.
Although the damage can still be outhealed thanks to the recent buffs, but this cannot last very long due to mana issues. Therefore the tough challenge is to sustain the mana long enough until the boss dies, and DPS high enough so that the boss dies before mana runs out. BiS gear or close to BiS gear is definitely needed here, but aside from that the management and rotation of defensive cooldowns is also important. Some defensive cooldowns can be used twice throughout the fight if rotated properly. For this particular kill. I rotated my cooldowns so that both Dispersion (glyphed) and Desperate Prayer can be used twice.
Since there aren't any mechanics to this boss, success at this fight all comes down to playing your class well and use defensive cooldowns to their maximum benefit.