Ok. So if you leave a game, you are sent an AIDS needle. Additionally, you get +1 disconnect AND are forced to play as Kongor next game. You can't leave his lair, except when a rune spawns. You must run directly to the rune (still as Kongor) and guard it. If you do not do this, then HoN immediately shuts down and formats your hard drive and injects you with the AIDS needle.
There is no such things as perfect build for Arachna. Its all depends of the situation. Lets refer to OP question.
1. Helm boost Arachna low HP for her to survive the initial jump (Lvl 25 Arachna HP with no item is around 1.6 k very low not to mention her base movement speed is 295). Its the same reason why you build Helm for Magebane.
2. Then tablet can be used immediately to escape even if you were silenced. (Bonus: tablet can be used to cancel enemy TP if the were ulted.)
Steamboots/post haste, geobane, shrunken, symbol of rage, savage mace/riftshards and wingbow gives you a nice mix of dps and HP.