As has been said, carry a tp scroll and tp away the minute you see her if you're alone. If ganking her, team up and stun her to death, she has paper HP so it doesn't take much. Spider only hits one hero so it's not too powerful in teamfights. If she is REALLY giving you trouble buy a void talisman, arachna immunity.
DEFEND ISRAEL, NO MATTER THE COST
1. Warbeast - max unslowable movespeed while in ulti form, and with hounds can one shot spider.
2. Zephyr - Wind shield is a good counter to all ranged carries. In case something goes wrong, just leap away.
3. Pyromancer - Can oneshot arachna at level 6, unless of course arachna goes tanky, but loses out in dps.
4. Plague rider - If arachna throws her ulti, you throw your own ulti and kill her in less than 3 seconds.
If you're going to talk about ganks with multiple heroes, there are a dozen other heroes I would rather have beside me than Arachna.
In short, 135g can, and usually does, counter Arachna. This alone is the main reason you don't see so many qq arachna threads anymore because after having it hammered into peoples heads for so long, people finally realised this and started doing it. I would suggest you do the same.
Don't put yourself in a position to be orbwalked/kited to death. Her health pool sucks, and she has no escape mechanism besides her slow. If you're mid against her:
Pyro and Witch can harass her out of lane with Wave/Graveyard.
Defiler can too with Wave, and her Ult beats Arachna's Ult.
Voodoo can combo her down with Stun/Curse/Ult, even through her magic armor.
Valk and Hag can blink away from her spider to the tower; Jav / Bat Blast take her down quick.
The only solo mid that really sucks against her from experience is Torturer, but he can just go side lane.
The Hunter- an original ganker/initiator.