Niche attackers - Tangela, Scyther, Parasect

Well, I always think the same with respect to Scyther, I have a specimen with very good IV and moveset but it requires a big investment of dust. As to whether I have a specialised attacker I can say my Frosth Breath/Blizzard Dewgong, wich use against Dragonites he is a sniper, the dragon killer!

PD: Parasect is a very good attacker, he is very fast at dodging

I've also considered powering up a scyther to use against exeggutors, but I'm still not 100% certain whether double strengths and weaknesses exist in this game. I've seen people say that they do, and that they don't. Anyway, fire-type Pokemon have worked very well for me against exeggutors. I haven't yet found a good scyther with the best attacking moves, so I haven't considered powering one up.

Against vaporeons, I use a vileplume. Once I manage to evolve a victreebel with razor leaf/solar beam I will be using that instead. Bulbasaurs aren't common where I live, but I would also use a Venusaur if I had a good one. I wouldn't consider powering up a tangela, since all the others I mentioned are better options.

Other specialized attackers I use are dewgong for dragonites, and jolteon for gyarados, slowbro, and sometimes lapras. I use charizard against exeggutor, since I don't yet have a good arcanine for it. I enjoy using wigglytuff against snorlaxes that use lick, since they are not very effective against normal type.

I powered up a 93 Scyther to 1757 with good attacks and use it almost exclusively against Eggsecutors. With the new team-training however, it's not necessary. It used to be that a 2200 Egg at the base of the gym meant you could no longer train it efficiently... that just isn't the case anymore. Jynx, Parasect, Scyther can all take down the Giant palm Tree well. Dragonite still tends to do it better (although the Breath animation gets in the way of seeing the Zen Headbutt for dodging purposes)

I use Butterfree to destroy Exeggutor. Those Bug Buzzes do insane damage, but you need to be super careful not to take a Psychic or Solar Beam to the face since Bug Buzz's animation is very long (~4 secs).

Tangela Parasect Scyther and Butterfree are all usefull but i wouldn't power any of them. Try to catch wild one with higher CP and right moveset and keep the smallest one for training gyms or attacking weak gyms

So here's the issue: niche attackers are, well, niche. Assuming that you have a finite amount of stardust, I think it's a waste to power up a pokemon like Scyther when you could power up a pokemon like Arcanine instead. First of all, while yes, Scyther's moveset is better for taking out Exeggcutor, the difference isn't that large: bug attacks do 1.56x damage while fire attacks "only" do 1.25x damage. Second, Arcanine has uses beyond just taking out Exeggutor - he makes a good anti-grass, anti-ice attacker and can be deployed at the top of a gym to get you several days' worth of gold. Third, niche pokemon tend to be very squishy - one charge move and they're gone. I'd rather not risk the fate of the battle on the stability of my Internet connection (or my sluggish dodging fingers, especially once it gets cold out)!

tl;dr - Powering up tier 1-2 pokemon will give more overall bang for your buck than niche 'mons.

my 1000 tangela takes out 2000+ vaporeons with roughly half HP left. I try to dodge every attack though. hope that helps