On a one on one fight to the death, the Alien creature would win because it's ability to coil up into a fetal position allowing the exospinal armor protection and the collapsing of limbs and torso to repel any attacks. From the fetal position the Arachnid would only be able to kick the Alien around like a soccer ball. Once the Alien found itself underneath the Arachnid, the Alien would propel itself straight upwards and use the inner telescoping jaws to rip into the Arachnid's central body cavity. The Arachnid would try to back away from the Alien allowing the Alien to use the tail as a cleaver through the Arachnid's multipile legs.
Once the Arachnid was totally defenseless, the Alien would drag it to the nest and let the face huggers impregnate the Arachnid. Many hours later after gestation, a new breed of Alien hybrid would emerge. The Octo-Alien.
As much as they are the toughest of science fiction killer creatures. They still don't hold a candle when compared to the true Top Dog of this genre. Spaceballs!!!!!!!!!!!!!! Oh my, what's that crawling out of that nose? Good lord, it's Spaceballs. There goes the planet. (said in a british accent)
