She used to be my favourite Hero, back when she was interesting to play.
And I say that mainly because the difficulty of playing Spectre has been reduced to almost zero. It used to be hard to farm with her; yes, she was the late game monster she is now, but it was actually hard to reach that stage. Then, we got huge kill bounties/AoE gold, Desolate that works on creeps, infinite Realities, more passive gold per second, etc etc. She can farm so fast, that getting Radiance is a luxury; she doesn’t even need it anymore.
Anyway, having played that Hero a lot, I can point out a few of her weaknesses here and there.
I don’t want this to be a “finish the game early” suggestion, for a couple of reasons:
1. This goes for most late game carries. Melee agility Heroes that deal a billion damage with one click 50 minutes into the game. I’d like to talk about Spectre specifically, and not any other carry that may replace her next patch.
2. If you’re reading this, you’re most likely a pub player. Finishing early requires a certain cooperation that you’ll rarely find in pubs. It doesn’t matter if you explain it with a thousand different ways, your  teammate will go AFK jungle Lifestealer and get his Midas. Welp.
So, I’ll leave the lane crushers/early pushers out of this, and talk about some Heroes you can pick against Spectre, when you feel all alone against her, and do a bit better.
 : Shadow Demon’s skillset is really made to give Spectre a hard time. Disruption means you create two of her illusions with all their benefits: Desolate, Diffusal manaburn, Radiance. Soul Catcher is a great counter for tanky Heroes. Your Purge makes sure she can’t just run away from your combo. Try it and you’ll be amazed at how effective it is.
 : Morphling would’ve been a really good counter if Radiance was still a must-get on Spectre, but he’s still worth a mention, since with his Replicate can create an illusion of an enemy that’s as tanky as the original and lasts for a long time. You can also use it on the Haunt illusions and it’ll be exactly the same.
 : So, why this guy? Phantasm is something that can cause Spectre a lot of problems. Besides the ability to push towers fast, it works both as a high-dps mechanism and a Desolate counter; jumping a spectre with 4-5 illusions that deal full damage is something she can’t fight back, especially when her Desolate doesn’t work.
Here’s a match with Illidan playing Chaos Knight against the infamous Spectre picker, Badman.
https://www.youtube.com/watch?v=6CjVkRtsOg0
 : I like Slark against Spectre for a variety of reasons. First, he’s a natural Shadow Blade into Silver Edge carrier, allowing him to roam around the map relatively early, not giving space to the enemy to farm, as well as taking away her Dispersion and Desolate. Shadow Blade also allows him to go invisible and avoid a lot of Haunt damage; and Shadow Dance is one of the few skills with which you can manfight a Manta+Diffusal Spectre. Essence Shift is also a great tool against tanky Heroes, since it takes away stats directly (each hit means -1 Str = 19 HP and -2 Agi = 2 damage, 2 attack speed and ~0.3 armour).
Always go Silver Edge against Spectre, it makes a world of a difference.
 : Alright, so. I haven’t actually tried this, but I thought I’d mention it because it seems good in theory. You have a skill that creates enemy illusions and makes them attack themselves (see a pattern here?). Being a ranged carry means you suffer less damage from Dispersion; and again, having your own illusions means you can avoid Desolate. His ult, Sunder, if timed and placed correctly, it’s a real pain for Heroes who need to be in the middle of a fight.
But, I wouldn’t put my hand in the fire for him.
 : The bane of melee carries. His Craggy Exterior makes it a nightmare to right-click him up close, but that’s not all. A high burst damage combo means he’s a threat for weak early game Heroes; cleave through Aghanim’s can clear illusions pretty fast; and the potential to siege towers with freaking tree can end games early.
And a special mention:
 : Do you have fingers to spare? Are you a good Meepo player? Spectre can’t fight you and she can’t run away from you.
And above all: patience. The next patch will come soon. Ish.
However, Spectre has always been a powerful Hero. She’s been considered broken quite a few times, perhaps more than any other Hero. Keep the above guys in mind and make your life easier.