Valkyrie (1/8/17/maximum)

Clvl: 30
Prerequisites: Dodge, Inner Sight, Avoid, Slow Missiles, Evade, Decoy.
Effect: Summons a powerful valkyrie warrior to fight by your side.
Properties: Minion, life scales with difficulty level, casting speed is subject to faster cast rates, uses magical and rare equipment with attributes of a level equal to (25 + 3{slvl}).
Spell Timer: 6 seconds.
Synergy from: Decoy: +20% life/slvl, Penetrate: +40 AR/slvl, Critical Strike: as per skill description, Dodge: as per skill description, Avoid: as per skill description, Evade: as per skill description.
Synergy to: None.

The valkyrie represents one of the most drastic changes to the amazon class as a whole, so read carefully.

The valkyrie ends most vulnerability problems a bowazon will have with tougher targets by granting her a remote shield. The valkyrie is a bowazon's most useful skill; however, I've learned to not rely on her to protect my bowazon all the time, but to use her in conjunction with my hireling, Slow Missiles, and Decoy. A quick look at her synergies should give an astute viewer a good idea of this minion's potential: imagine a high-level spearazon with 2,500-4,000 life with your bowazon's base passive skills, sacred armor and a war pike, and that's what a valkyrie is. Along with the clay golem and shadow master, the valkyrie sits in the enviable position of being one of the best minions in the game. Gaining your first valkyrie is like having sex for the first time: once you've had it, you can't imagine the rest of your life without it. A valkyrie is absolutely mandatory for any bowazon build, and there's absolutely no good excuse to not have at least one point invested in this skill.

Point investment depends purely on personal preference; mine is for a high-level valkyrie…eventually. You only need your valkyrie to be tough enough to last at least seven seconds against most of the monsters you'll run into: if she dies or hits the red after most encounters, you'll definitely want to invest more points because the valkyrie is usually the only thing standing between you and a dead character, and you don't want her spell timer to be down when she bites the dust. That means the faster you run through the game, the more urgent the need for a higher-level valkyrie. Valkyrie life also scales with difficulty level (as noted by Chippydip). In short, the tougher you want your valkyrie to be, the more points you invest in Valkyrie, Decoy, or both.

Important breakpoints for the valkyrie are slvl-8 (when she gains her lance) and slvl-17 (upgrade the lance to a war pike).

You can still use the valk combat-drop — casting a minion in the middle of a group of monsters — and I would heartily recommend doing so, but because of valkyrie's mana cost, quirky path finding, and six-second casting timer, you aren't going to find yourself doing it too often. It depends on how many crowd-control options you've given your bowazon: if you have…say, a merc, decoy, valkyrie, freezing arrow, and strafe, you won't be repositioning your valkyrie too much because you have so many other options to depend on.

It's just that simple; what's not simple is balancing out your bowazon's defense via valkyrie with her offensive potential and your bowazon's offensive skills (which is taken care of on a case-by-case basis). Another reason to invest in your valkyrie is to upgrade her items, which brings to attention to ability to actually kill things, something she's never been able to do before.

When a valkyrie is summoned, she automatically receives weapons and armor. These items receive attributes of a level equal to (25 + 3{slvl}), so a slvl-5 valkyrie will be using a clvl-40 spear. As of v1.10, the valkyrie can spawn with…

— Fuzzbox

…with many thanks to Fuzzbox, adeyke, and HunkyDory, who found this all out. According to Ruvanal, valks don't get any new types of equipment after slvl-27. Unless listed otherwise, all equipment is magical. Eurynome went into more detail about this.

— eurynome

— Arreat Summit

Like all minions, a valkyrie takes extra damage from Andariel, Duriel, Mephisto, Diablo, and Baal. A hot key for valkyrie is highly recommended. For your information, I named my valkyrie "Lenneth."