mara definitely > locket, 2 all skill and 30 resist just too good to pass if you using void. bk/soj/arach is optional because they are easy to find, I'd swap them for slings and gheed once you get all the bis gear. presume you get perfect everything you can could get to -75% magic resist, thats almost 7k magic explosion damage right there. for those who dont understand what this mean, essentially mean you are firing 5 projectile each that will have 100% chances to erupt and have an aoe impact of 4 yard while doing 7k magic damage. necro's wet dream right there
ye but keep in mind if you prebuff the +skills don't do as much. I mean, reducing the mana cost on teleport and +2 battle orders is probably the biggest benefits from the skills, otherwise its just a resist amulet.
excluding skill shrine / consumed hephasto you could get up to level 58 hex purge, more realistically 52-53, then wind up with +54% magic / -73% resist
hard to calculate the exact damage, but I see p8 pit lord in river of flame lose about 80-90% of its hp from the outgoing missiles alone (not including boomerang return). And that's about 64k hp / 0% magic resist, so figures it has to be about ~55k hp lost, about ~32k magic before -resists, about 6.4k per explosion
although even with a blessed aim merc it might be worth using ravens or even angelics set or something crazy like that