Just another first timer here, but here's what I've been doing so far.
Originally I decided to go with Magic Arrow (MA), but I've now decided to instead get a Witchwild String. Currently I have 1 hardpoint in MA as prereq, and a bit from gear (this will get lower, though, since I'll replace the Vipermagi with something else).
My Windforce takes a while to whittle down physical immune bosses in A1 or spectres in Arcane Sanctuary with this low level MA; it works a bit better with Itchies in A2. So I really want to get that Reapers, and the WWS (which I'll upgrade). My *impression* is that MA is viable, but it needs some more skillpoints (hence WWS on switch

) or quite a bit more physical damage than I'm dishing out, and preferably Amp Damage or Decrepify to take care of the "basic" physical immunes, leaving only the hard PIs to the backup skill --- I have't tried the Decrepify yet, though (best case scenario still leaves immune monsters with 90% resists, which means your normal arrows do about the same damage as a lvl 10 MA unless the monster has magic resistance --- but with Decrep I'll be able to Strafe, thus pumping out arrows at insane speed for a lot more damage/time).
Actually, I don't really know whether it's the magic damage from MA, or whether it's really the Valk doing the damage

That's a bit I don't really like about the Valk: I have no idea what gear she's wearing, except for the obvious mods (armor color and ctc effects going off) --- and it's changing everytime I recast her, which happens frequently since she's always off on a sightseeing tour when monsters are coming towards me.
As for the Valk... I'm planning to max it. Still having problems with my merc (currently leaving him dead until I get the Reapers, and will try again then), and even if I do get the merc to survive the Valk will still be 50% of my minions --- and the only recastable one, which pretty much makes her a replacement for Gumby, except that she isn't quite that good as tank and has an annoying timer, but at least she does some damage on her own instead.
The "magic number 17" is because that's the level where her weapon is upgraded to a rare war pike (IIRC).
I found Decoy to be useless, but I only spent the 1 point in it to get to Valk, so maybe more would make it useful. Not planning on doing this, though --- if I use Decoy it's mostly to see whether something happens upfront, in which case I'm replacing it with the Valk...
I've also used the Decoy on the nightmare baalruns to possibly buy a "splitsecond" to see where the pack is appearing so I could cast the Valk at the correct place.
And believe it or not, I also like to fire off a Strafe to see if monsters are nearby --- it seems to have an auto-targeting range that extends beyond the visible area. Which is sometimes annoying as well... but I guess with playing time comes playing practice and experience. I've already started to throw freezing arrows between my strafes...