Work in progress
Semi-casual Eidolon build
tl;dr: Beginner friendly, just hit 2 and make grass around the boss. Then stand in it.
This build's main purpose is to [Protect] your squad from magnetic and other status effects by setting up a couple Hallowed Grounds around the Eidolon.
I rarely use [Oberon] in Eidolon fights anymore. [Harrow] (guide here, also further going into detail about other aspects of the Eidolon fights) can provide status protection too. But this requires a) [Harrow] and b) a more complex playstyle and deeper understanding of the boss fight.
Depending on your squads setup and proficiency, you can modify this build as you wish.
This basic setup does a good job on beginner or random squad level.
If you want to use Augment mods, you can replace Stretch/Augur Message by Phoenix Renewal/Smite Infusion:
[Phoenix Renewal]: Can save your or your squad mates' life every 90 seconds. In my opinion, [Oberon] is not the best Eidolon healer. Since his Renewal is a toggle ability, your only way of restoring energy while it's active is to pick up energy orbs. Having all players and lures in range drains a considerable amount of energy. For [Phoenix Renewal] to do its work, you'd have to keep it running nonstop. If you want to play the healer role, I'd recommend Trinity.
[Smite Infusion]: Recommended! Try replacing [Stretch] with this mod. Since the rework of these types of Augment mods, this is actually viable and easy to apply to everyone, including yourself (Hold ability hotkey to cast on self). This can certainly pump up your squad's damage output, especially in a beginner or random team. Scales with your Warframe's modded strength, range and duration.
Steel Fiber is not the ideal choice here, but since everyone has it and this build's purpose is to be beginner-friendly, it'll do the job. Use [Adaptation] instead if you already own it. If you don't need the extra damage reduction, try Intensify (stronger heals + buffs) or Constitution (reduces energy consumption + better knockdown recovery).
Power Drift's job is to occasionally protect you from knockdown effects as well as to increase your healing capabilities and - in case you're using Smite Infusion - buff strength. Strength is useful but not mandatory for this build. If you get knocked down a lot, try Handspring.
Aura: In most squad setups, your go-to aura for Eidolons is [Corrosive Projection]. You can add another regular Forma 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 not use Oberon anyways.
...are recommended but never required. Use what you prefer/own. In case of doubt, put an Arcane Nullifier and Momentum on any frame/build you'd like to use for Eidolon hunts.
More Eidolon builds:
Questions? Feel free to message me ingame or via Discord. :)
Warframe tips and beginner's guide here.