the thing with arachna is that she's very similar to viper..there are lot's of builds for viper...
as for arachna, if there are lots of casters and disablers, i would choose to have the second skill over stat..
my usual build: vanguard,hood,phase boots/treads, then after that covert to damage..like lothars/radiance/mkb..etc
the advantage of this one is that ur like the secondary hitter..you can tank damage and slow the enemies that your primary end gamer is hitting..sounds easy huh?
if you i'm the only farmer in the game then go for assassin's shroud first(lothars).then manta so i could have a higher hp and wouldn't be an easy target..your aura also applies to your illusions so i would also say that it would be considered as a damage item..then you can do any damage item that you want...well of course you could use wingbow and thunderclaw etc etc
i will write 'premium' guides in my blogs http://fallenpaladin.wordpress.com/
soon. so i hope you guys check it out next year..