This was deemed as one of the easiest naxxramas bosses and was usually the first faceroll target of any naxx pugs back in the day. However from a shadow priest solo standpoint, this is far from the case.
The main challenges to overcome in this encounter are Locust Swarm, Impale as well as the adds. Together, these three boss mechanics makes the encounter pretty much a living hell for a shadow priest.
The Impale is a really annoying ability that knocks the player upwards, disrupting all spell casts, followed by around 25-28k worth of fall damage (depending on max HP). Luckily for a priest, this can be avoided with levitate. It is also useful to use up a GCD whilst in the air (such as PoM, renew, PW:S or DP) since there is plenty of time before the need to cast levitate arrives. Nevertheless, Impale is still really annoying since the boss (or any adds that may be present) can still hit the player while he's in the air.
The adds themselves don't do that much damage, but they place a poison DoT on the player that stacks, making them a top priority kill target. This means that while the add is up, healing should be minimized and all GCDs available should be used to burn the add. This is really troublesome when health is low by the time the add spawns. Occasionally, small scarabs may spawn from the corpse of these adds, and these can be easily dealt with.
Being close to the boss during Locust Swarm is an instant silence on the player as well as a stackable DoT, and this is usually avoided by kiting. However despite the 40% movement reduction on the boss during this mechanic, the player still requires a speed buff of some sort to outrun the boss. Additionally, an add also spawns during Locust swarm which adds to the difficulty of overcoming this mechanic.
These three mechanics combined makes this encounter extremely mana intensive. So the usual mana return strategies must be used as frequently as possible.