This Tenet Spirex is actually very good for Eidolons. I have gotten multiple limb one-shots, and never more than a 2 shot if you have all of your buffs up. Its Small AoE means that it can occasionally kill both Vomalysts with 1 shot if they are grouped up. Unfortunately, it is not hitscan, so no water limbs, but you should have [Rubico] or another sniper anyway.
Some of you may be questioning [Ice Storm] in this build instead of [Magnum Force]. I find that with [Magnum Force] the accuracy is simply not good enough, and causes some of my multishot to miss, leading to lower damage than if you simply had [Ice Storm], which also gives the [Mag] capacity for if you forget to reload since the [Mag] is relatively small.
The Spirex also has a riven Disposition of 0.90, so you could get a pretty good Riven that would enable this to consistently 1-shot limbs. A CC CD MS with -zoom or other harmless neg would probably be best.