1: Pros and Cons of Grass-type Pokémons
Pros:
- They hurt and resist Ground and Water, 2 common types.
- They are effective against rocks.
- They resist Electric and themselves.
- They have a variety of useful moves such as Leech Seed and Spore.
- They are usable in Sun Teams.
- They have generally a good movepool.
- SolarBeam, Energy Ball and Giga Drain are example of nice grass attacks.
- Usually bulky and pretty defensive.
- Resist Leech Seed and Spore.
Cons:
- They are ineffective against Fire, Grass, Poison, Flying, Bug, Steel and Dragon. This is a lot.
- They are weak to 5 types.
- They are usually slow, and lacks HP.
- Easily sweeped.
- Most of them are useless.
- Dominant in NU, proof of their uselessness.
- Contrary Serperior still unreleased.
2: Are them a threat ?
Mostly not. There are really a few usable Grass Pokémons:
- Venusaur
- Tangrowth
- Roserade
- Abomasnow
- Whimsicott
- Ferrothorn
- Virizion
I will help you on dealing with these specifically.
Venusaur
It can get dangerous, as Thick Fat, when Mega-Evolved, it loses its Fire and Ice weakness. The best counter is Mega-Alakazam, who OHKOes it with Psyshock easily. Trace will copy Thick Fat, leaving Alakazam resistant to fire and ice for free.
Alakazam @ Alakazite:
Trait: Trace
EVs: 252 Sp.Atk / 252 Speed / 4 HP
Timid Nature
- Psyshock / Psychic
- Shadow Ball
- Dazzling Gleam / Energy Ball
- Focus Blast / Charge Beam
Tangrowth
A physical wall with Regenerator. Hard to battle, if switching is enough to get back health. But I think it has a wide open weakness to special attack. I would say Gengar is the best counter with Sludge Bomb. And moreover, you block Tangrowth with Shadow Tag, making it unable to use Regenerator.
Gengar @ Gengarite:
Trait: Shadow Tag
EVs: 252 Sp.Atk / 252 Speed / 4 HP
Timid Nature
- Sludge Bomb
- Substitute
- Disable
- Shadow Ball / Focus Blast
Roserade
Roserade sets up Spikes and Sleep Powder because now, it is legal. So it is a bit harder to battle than before. Still, fire, psychic and flying Pokemon remain the best solutions. Search for one. I think Prankster Taunt may shut it down, so Tornadus is a counter, as it can also hit with Hurricane. No sets here.
Abomasnow
Do not think a little bit, just 2 seconds and you understand that any Fire Pokémon can deal with it. Houndoom is an example.
Houndoom @ Houndoomite:
Trait: Solar Power (Usable with Ninetales to win the weather war.)
EVs: 252 Sp.Atk / 252 Speed / 4 HP
Timid Nature
- Flamethrower
- Dark Pulse
- Sludge Bomb
- Shadow Ball
Whimsicott
Whimsicott can not be demolished by dragons, courtesy to its new typing: fairy. The above Gengar is the perfect counter to Whimsicott.
Ferrothorn
Everybody agree with me that Ferro is weak to Fire, no ? Then, what do you wait ? Houndoom, above, is enough.
Virizion
This one is demolished by fondant's Skarmory set. And Honchkrow.
Hope I helped !
In general, Poison, Ice, Flying, Bug and Fire are enough against Grass.