With access to two useful status-inducing moves, a slew of handy resistances, reliable recovery, and respectable defensive stats, Parasect can act as a superb status spreader. Parasect can switch into many common Pokemon, such as Milotic, with ease, which gives him plenty of opportunities to switch in and spread status. Though Parasect's numerous weaknesses can cripple him in this regard, the opponent cannot switch in Pokemon who can target Parasect's weaknesses easily due to the threat of Spore and Stun Spore.
Spore is this set's main selling point, forcing the opponent to be constantly wary of switching Pokemon into Parasect haphazardly. It should be noted that Parasect may not wish to use Spore immediately; though it may seem tempting to do so, the constant threat of Spore will pose a psychological threat against the opponent, and you may wish to scout the opposing team for any Sleep Talkers before using up Parasect's precious Spore. Stun Spore allows Parasect to act as an effective paralysis spreader, and, since Stun Spore's paralysis is only negated by Pokemon with the Limber ability, Parasect is almost guaranteed to paralyze an opposing Pokemon once he gets in safely. Stun Spore is also the safer option to use immediately, since it can catch a threatening Pokemon like Arcanine on the switch. Synthesis gives Parasect a means of reliable recovery, and further adds to Parasect's nice bulk. Finally, Seed Bomb rounds off the set by providing a reliable STAB move that still hits fairly hard off of Parasect's respectable Attack stat, despite the lack of EV investment. However, X-Scissor can be used over Seed Bomb to hit Grass-, Psychic-, and Dark-types harder, though Seed Bomb's better type coverage means that it is generally the better option.
The EVs of this set go towards increasing Parasect's special bulk. This is because many of Parasect's resistances, such as those against Water-, Electric-, and Grass-type attacks, are more commonly seen as special attacks, which makes investing in Special Defense more worthwhile than investing in Defense. For example, a specially defensive Parasect takes relatively little damage from Milotic's Ice Beam, which allows him to switch into Milotic with ease; it also allows him to survive some strong super effective attacks, such as Life Orb Venusaur's Sludge Bomb. Also, despite being specially-based, Parasect can still take Choice Band Close Combats and Earthquakes when need be. However, a physically defensive spread isn't a terrible option, as it does allow Parasect to check Pokemon like Dragon Dance Feraligatr.
This set has numerous options that can be used over the suggested ones. While Dry Skin is usually the better ability, Effect Spore can be useful to allow Parasect to spread status by simply switching into a contact move. The immunity to Water-type attacks that Dry Skin provides is generally too useful to pass up, however. Knock Off can be used in Stun Spore's stead to remove opposing Pokemon's items, but the paralysis offered by Stun Spore is generally more useful. Finally, Aromatherapy is a useful option to cure Parasect and his teammates of status, but it's difficult to find a place for it on this set.
Since this Parasect can act as a very effective status-- in particular, paralysis -- spreader, slow, powerful Pokemon work well with him. Rhyperior is perhaps the best of these, as he resists all of Parasect's weaknesses except for Ice-type attacks, and in exchange, Parasect resists Rhyperior's nasty weaknesses to Water-, Grass-, Ground-, and Fighting-type moves. However, Aggron, Hariyama, Azumarill, and Ursaring are all good choices as well, though they unfortunately don't have the almost perfect defensive synergy that Rhyperior and Parasect share.
Defensively, bulky Water-types can be useful, since they resist Parasect's weaknesses to Fire- and Ice-type attacks, and the combination of Water- and Grass-type Pokemon is a fairly decent defensive tandem. Milotic, Blastoise, and Lanturn are great bulky Water-types who can also complement Parasect in other ways. Since Parasect is very prone to status, having a cleric on your team can be useful; Clefable, Chansey, and Lanturn are great Pokemon to fulfill this role. Due to his Stealth Rock weakness, Parasect appreciates Rapid Spin support, as Stealth Rock will cause some attacks that would otherwise be manageable to 2HKO or OHKO him, and is generally extremely annoying for Parasect to deal with. Hitmontop, Blastoise, and Donphan are all excellent Rapid Spinners, since they can all deal with spinblockers who may attempt to halt their attempts to Rapid Spin.