Paras can be found in Mt. Moon, and is uncommon (5%) on 1F, is the only possible encounter (100%) on B1F, and is relatively common (15%) on B2F. It is relatively common (15%) in Areas 1 and 2 of the Safari Zone. Parasect can be found in the Safari Zone, and is a relatively rare encounter (5%) in the Entrance Area and in Area 1.
Paras and Parasect are Pokémon that look pretty good on paper. They're the only Pokémon with Spore available to them in this game and they have a solid Attack score along with acceptable bulk. However, the bad far outweighs the good in this generation. With Grass still being a special-based type and Bug having next to no good attacks, Parasect is likely to be stuck with moves that just don't work on it, and it doesn't get to take what little advantage there is of its horrible typing. If you are going to use one, be careful not to let it fight... well, a lot of things, really.
Rival (Cerulean City): Do you really need to be told that you should stay away from Pidgeotto and Charmander? No? Good. Abra can be handled by literally anything with a damaging move, but stay away from Rattata, as STAB Hyper Fang is quite strong, and your ability, Effect Spore, may paralyze or poison it, which would trigger Rattata’s Guts and make it even stronger.
Gym #2 - Misty (Cerulean City, Water-type): As long as you have a Grass move (Bullet Seed via TM09 that you find in Mt. Moon is the only one available at this point), Paras is good for resisting Water Pulse and dealing damage. However, there are much better options available for this, namely Bellsprout or Oddish.
Rival (S.S. Anne): It's not stellar against anything, but if you're still carrying Leech Life at this point (which you should be, as it’s the only Bug-type move you’ll ever learn), that might be useful against Kadabra.
Gym #3 - Lt. Surge (Vermilion City, Electric-type): Parasect resists Electric and can learn Dig, but there are a lot of other Pokémon that do the same thing. There is one thing that gives you an edge, though: if you either wait until Paras is level 25 to evolve it or overlevel Parasect for this Gym, you'll have Spore at this level, which is good for shutting down Raichu's Double Team.
Rival (Pokémon Tower): If your rival has Exeggcute, that's pretty much all Parasect will have to do here. You could try to also take down Kadabra or Wartortle with your STABs, but be careful of Kadabra’s Confusion, as it might be able to 2HKO you if one of the hits is a critical. You could also try to put Gyarados to sleep with Spore and then switch out to someone who can actually handle it.
Giovanni (Rocket Hideout, Ground-type): Use your Giga Drain TM on Parasect? You're golden! Just don't keep it in on Kangaskhan unless you want to try putting it to sleep.
Gym #4 - Erika (Celadon City, Grass-type): It's an option, but not really a great one. You can have Aerial Ace by this point, so you can try to spam that, but due to Aerial Ace’s low power, you won’t be scoring many OHKOs. However, you do have a double resistance to all of the Grass-type STABs being tossed around in this gym, so you can use that to your advantage. Be careful, though, as Victreebel and Vileplume can nail you with STAB super-effective Acid.
Gym #5 - Koga (Fuchsia City, Poison-type): HAHA no.
Fighting Dojo (Saffron City, Fighting-type): You should be safe here, especially if you have Aerial Ace. You resist all of the dangerous Fighting-type moves that these Black Belts use, and Aerial Ace will do a number on most of the not-so-bulky ones (Hitmonlee, Mankey, Machop). Hitmonchan does have Fire Punch, though, and you SHOULD be wary of it despite its abysmal Special Attack; it is still a double weakness after all.
Rival (Silph Co.): Same as last time, really. Alakazam is no longer a threat, since its only attacking move is Future Sight, so you can take that down with a few Leech Life if you want.
Giovanni (Silph Co., Ground-type): Still good for Rhyhorn if you have Giga Drain. Kangaskhan isn't as threatening in this battle, so using it against her is an option for its bulk.
Gym #6 - Sabrina (Saffron City, Psychic-type): You should PROBABLY stay away, since Leech Life is weak and Psychic is incredibly strong. Aerial Ace does give you something for Venomoth, though.
Gym #7 - Blaine (Cinnabar Island, Fire-type): RUUUUUN! RUN, FOR THE LOVE OF GOD, RUUUUUUUUUUUUUN!
Gym #8 - Giovanni (Viridian City, Ground-type): Giga Drain has an advantage against three of his Pokémon, but watch out for Rock Blast. You should be able to outspeed and OHKO the Rhyhorns before they nail you with Rock Blast, though, and Dugtrio can’t touch you outside of the weak Slash.
Rival (Route 22, pre-Elite Four): Rhyhorn and Exeggcute are no problem, and Blastoise has no Ice moves. Stay away from the rest unless you really need to put stuff to sleep.
Elite Four Lorelei (Indigo Plateau, Ice-type): You can take on Cloyster. That’s it. Cloyster is the only one who doesn’t have an Ice-type move, and STAB super-effective Giga Drain should be able to score a OHKO on it, considering its abysmal Special Defense. Stay away from everything else, as all of Lorelei’s other Pokémon either have Ice Beam or Ice Punch.
Elite Four Bruno (Indigo Plateau, Fighting-type): Surprisingly competent, actually! Aerial Ace and Giga Drain gives you super-effective attacks for every member of his team. The Onixes are too weak to do any real damage to you, but you’ll need to watch out for Rock Tombs from Hitmonchan and Machamp. Seeing as it’s a 50 base power non-STAB move, though, you’re almost definitely not going to get OHKOed, leaving you time to put them to sleep with Spore and start hacking away with Aerial Ace.
Elite Four Agatha (Indigo Plateau, Poison-type): Everything's a Poison-type. Enough said.
Elite Four Lance (Indigo Plateau, Dragon-type): Well, let's be honest...the Dragonair are kinda weak. Pretty much anything can beat them. Stay the hell away from Aerodactyl and Dragonite, as both have Wing Attack, and be cautious against Gyarados. Only stay out against it long enough to use Spore.
Champion Rival (Indigo Plateau): Pretty much the same as the Route 22 battle, but watch for Alakazam’s Reflect.
Post-Game: Yeah, Leech Life is not going to do anything to Mewtwo, assuming you even have it (which you shouldn't at this point). However, if you plan on rematching the League, Parasect can still be of help somehow. A bit.
Well, Spore (level 25 for Paras, level 27 for Parasect) is a given and is pretty much Parasect's only saving grace in this generation. A lot of the important battle analyses above can have a lot more Parasect in them if you make use of Spore, but they're assuming you don't want to take too many risks. As well as Spore, Parasect is a good candidate for Aerial Ace and Giga Drain (though you'd probably be better off giving Giga Drain to something else, since Parasect learns it at level 51), and it learns Slash at level 35. Dig is also an option, being purchasable rather than just being a one-time deal. During the postgame (assuming Parasect survives that long), Swords Dance and Sludge Bomb become options, but... well, do you really want to use those on a Parasect?
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Paras Line Ratings
What Nature do I want? Anything works, really. You don’t want one that lowers Attack, as that’s Parasect’s best stat and most of its best moves are physical. Special Attack-lowering Natures cut the power of Giga Drain, one of its most useful moves. Natures that hinder its bulk aren’t too great, as Parasect really relies on its Defenses due to its awful Speed. Come to think of it, any neutral Nature or any Nature that lowers Speed will probably be your best bet.
At what point in the game should I be evolved? Probably either shortly before or after you fight Lt. Surge.
How good is the Paras line in a Nuzlocke? Pretty awful, honestly. Two double weaknesses to extremely common attacking types in Fire and Flying, as well as a weakness to omnipresent Poison-type moves and good-but-not-great defenses make Parasect pretty difficult to keep alive at times. It has an extremely limited movepool, and it doesn’t have any decent Bug-type STAB moves to use off of its good base 95 Attack stat. Its main use is as a Spore-abusing sleeper, but Butterfree, with its Compoundeyes Sleep Powder, undoubtedly does that job better. Parasect may be bulkier than Butterfree, but it’s also slow as hell and doesn’t get as many useful moves as Butterfree. It can be a useful Pokémon to have around, but when you’re using something that’s outclassed by Butterfree, of all things, you can probably do better.
Weaknesses: Poison, Bug, Ice, Rock, Flying (x4), Fire (x4)