Arachnids are lone hunters - they patiently wait for their prey to kill it with one swift strike when it appears. The same is true for the arachas, a huge creature that took a liking of the riverside forest, becoming it's undisputed king. A ruler who does not tolerate other hunters on it's territory. Including witchers.
Arachasae are large, slow and protected by a durable armor. The carapace, as the witchers call this armor, is especially tough from the front, so it is much easier to wound the creature from the side or the back. The arachas1 charges make an excellent for that - one has to evade the charge at all costs by stepping out of the beast's way, and then make one's blow. Without doubt it's best to use the strong style then.
The arachas has no fear of poison, and not much fear of fire. It's primitive nervous system barely reacts to wounds, and it's incredible vitality allows it to take even great wounds. The beast will heal them after the fight anyway, all the while digesting it's prey.
All said and done, the arachas is a bug, so one's blade should be coated with the Insectoid Oil before fighting it. The monster's susceptibility to this blade coating is probably its sole weakness. The beast can easily all shrug off other witcher tricks, so common poisons and Signs are of no use, not to mention attempts to knock the colossus down.