WARNING: For players who have NOT finished The Second Dream and [War] Within quests, be aware that this guide contains mild spoilers.

This build is meant for Eidolon hunting with a casual (aka not fully optimized) squad and/or random players. If you're new to Eidolon fights, I recommend playing [Rhino] until you're used to the Eidolon's mechanics.

tl;dr:
1. Roar (3) will significantly buff your whole squad's damage. Cast before using your sniper rifle.
2. Cast Iron Skin (2), recast if it ran out.

Your primary task is to deal damage and to provide your squad with Roar's damage buff while staying alive. I don't like all these 8000% strength builds floating around in the internets. They are a) really expensive and b) have so little range that you'll never buff your team mates. (They are great if you wanna do solo stuff...)

Build optimization and variations:

• Feel free to modify the build to suit your playstyle.
• If you are new to Eidolon hunts, I recommend [Iron Shrapnel] instead of [Augur Secrets]. It enables you to recast your Iron Skin before it runs out. If you own neither of these, add some more duration, range, armor, or Adaptation.
• In most squad setups, your go-to aura for Eidolons is [Corrosive Projection] (+[Coaction Drift]). You can add another regular or Aura forma if you want to. But this kind of minmaxing would only be necessary if you're aiming towards tridolon speedruns - in which case you'd probably play a different frame. Dead Eye is a good alternative.
• Depending on your team setup, you might not want to use [Corrosive Projection] (and [Coaction Drift]) anyway. Your goal is to reduce the Eidolon's armor as much as possible without fully removing it. This way, your sniper rifle (modded with radiation and cold) will deal the maximum amount of damage.

Arcanes:

• Momentum (sniper reload speed) is extremely helpful!
• Arcane Nullifier can protect you from those pesky magnetic status effects.

--- Operator section --- (If you don't know what that is, it's a SPOILER!)

Focus schools

• I recommend utilizing the Zenurik school (Energizing Dash + Waybounds) for beginners/casual runs or Void Strike for more experienced players.
• Fully unlocking the Vazarin waybounds asap is highly recommended!

Amp

• If you're fairly new to this, build any modular Amp and gild it, it'll be better than what you're using right now.
• The ideal Amp would be a X-2-7 (Prism of your choice, Shraksun Scaffold, Certus Brace). Use the Shraksun alt fire to bring down Eidolon shields.

Arcanes

• Jam a Virtuos Strike into your Amp.
• If you can get your hands on Magus Vigor and/or Magus Repair, use them! They'll highly increase your survivability.

There's always some more potential to further minmax, but I like this build and its moderate amount of polarized slots, leaving room for variations and alternative configs.

More Eidolon builds:

• Chroma (damage)
• Harrow (support/damage)
• Volt (damage)
• Trinity (healer)
• Oberon (support/healer)

Questions? Feel free to message me ingame or via Discord. :)
Warframe tips and beginner's guide here.