Parasect is certainly an interesting and unique Pokemon. His odd typing grants him extremely useful resistances to Water-, Ground-, Grass-, Fighting-, and Electric-type attacks, and his defenses are decent enough to utilize these resistances effectively. His excellent ability, Dry Skin, allows him to switch into bulky Water-types with ease, and also slightly heals him at the end of every turn while in the rain, giving Parasect a unique niche on Rain Dance teams. However, none of the aforementioned factors are Parasect's main selling point. Parasect's most useful trait, by far, is his access to Spore, which gives him the ability to cripple any counter temporarily, almost always allowing Parasect to damage the opposing team in some way. All of these unique characteristics make Parasect stand out amongst both his Bug- and Grass-type brethren, giving him competitive value and usability.
Unfortunately, it isn't all good news for Parasect. Parasect's typing grants him horrible, crippling weaknesses to Fire-, Flying-, Poison-, Rock-, Ice-, and Bug-type attacks, most of which are common in the UU metagame. Parasect is also exceedingly slow, and is outsped by almost every Pokemon in the UU tier, except for Slowbro and Steelix (whom he speed-ties with). Due to these two traits, it's extremely easy to hit Parasect with a strong super effective attack once he switches in. In conclusion, Parasect, despite his many faults, can prove fairly useful to a team thanks to his unique assets.
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.Team Options & Additional Comments >>>
This set attempts to use Parasect's good base 95 Attack in the form of a bulky attacker. Spore is the main option on this set, as it's Parasect's main weapon against his counters. Seed Bomb and X-Scissor have decent coverage together, and are both boosted by STAB. Pursuit is a great attack to use alongside Spore; since Spore tends to force switches, Pursuit can smack the Pokemon who switch out for decent damage. Parasect can also use it to trap unsuspecting Ghost- and Psychic-type Pokemon. Brick Break is a usable option over either of Parasect's STAB moves, since it hits the Steel-types who wall both of his STABs for respectable damage. However, Parasect probably still won't be able to break through the majority of Steel-types of UU, so it isn't recommended. Synthesis is a decent option over Pursuit to give Parasect a form of reliable recovery, and it complements either item choice well. Life Orb gives Parasect's attacks more sting, but it comes at the cost of reduced longevity. However, with Dry Skin (and possibly Synthesis) to make up for it, Life Orb is a decent option, though Leftovers is always the safer choice.Team Options & Additional Comments >>>
Parasect's movepool, while not particularly diverse, does hold a few interesting options. Swords Dance, Agility, and Growth are all boosting moves that Parasect has access to, and he can use each of them with moderate success. Thanks to Parasect's respectable bulk and access to Spore, he doesn't have too many issues obtaining the boosts. However, in the case of Swords Dance and Growth, Parasect is simply far too slow and has too many common weaknesses to utilize these moves effectively, while with Agility, Parasect is too weak. If one uses Trick Room support alongside either Swords Dance or Growth though, Parasect has a much better chance at pulling off a successful sweep. In a similar vein, Screech is an interesting option to weaken the foe's Defense, and with Pursuit, Parasect can smack fleeing Pokemon for large amounts of damage. Unfortunately, like the aforementioned Swords Dance, Parasect is simply too slow to use this strategy effectively.
In terms of support options, Parasect has access to Aromatherapy, Knock Off, Light Screen, and Worry Seed. Aromatherapy can be used to clear status, and, thanks to Parasect's resistances and bulk, can be used with decent success. Sadly, Parasect often has trouble finding room for it. Knock Off is a great move that can remove key items from opposing Pokemon, but just like Aromatherapy, it's hard to fit it in on any of Parasect's sets. Light Screen can bolster Parasect's Special Defense further, and can be used alongside Counter to encourage foes to attack Parasect physically, allowing Parasect to respond with Counter. Worry Seed can remove important abilities from the opposing Pokemon, but changing the opponent's ability to Insomnia heavily clashes with Parasect's main weapon: Spore.
Checks and Counters
The main problem with countering Parasect is his ability to essentially remove one of his counters with Spore. Once Sleep Clause has been activated though, Parasect becomes much easier to deal with. RestTalkers such as Arcanine and Weezing can easily absorb Spore, and don't really fear being slept thanks to Sleep Talk. They also can easily threaten Parasect with their super effective STAB attacks. Alternatively, you can use a Pokemon with Vital Spirit or Insomnia to act as a decent switch-in to Parasect. Unfortunately, most of the Pokemon who have these two abilities are either frail enough to be overcome by Parasect's attacks or are weak to one of Parasect's STAB moves. Thus, out of the Pokemon who have these two abilities, Primeape and Noctowl are likely the best options to use against Parasect, as Noctowl is bulky enough to easily take Parasect's attacks while Primeape can threaten Parasect with Stone Edge, Ice Punch, or U-turn.
If you don't have either of these options, defensive Pokemon are decent switch-ins to Spore, since they should be bulky enough to survive being asleep for a couple of turns. Registeel is a prime example of this, since it's extremely bulky and resists almost all of Parasect's attacks. If Sleep Clause has been activated, Fire- and Flying-types easily wall Parasect. In particular, Swellow is perhaps the most dangerous out of this group, as he will often be burned or poisoned, granting him immunity to Stun Spore, and can easily shift the momentum in your team's favor with U-turn. Really, once Sleep Clause has been activated, any Pokemon who doesn't mind paralysis and resists Parasect's STABs will do just fine against him.