This is my build for Orphix missions, I don't think there is any tankier mech setup.

Goes together with:

• Ironbride build: https://overframe.gg/build/772612/ironbride/maximum-orphix-damage-ironbride/
• Fluctus build: https://overframe.gg/build/781838/fluctus/orphix-resonator-meltdown/
• Jade build: https://overframe.gg/build/781834/jade/veil-proxima-orphix-necramech-buffer-jade/
• Adarza kavat build: https://overframe.gg/build/781845/adarza-kavat/adarza-necramech-tuner/

What is going on:

• Jade's 2nd ability can give weapon damage to squad (and mechs) in AFFINITY range which is INFINITE in railjack missions
• Some aura mods work on mechs. Worthy Comradery works on mechs. Bonewidow while using her 4th - Ironbride hits Orphix weakspot from any angle and Orphix only has weakspot hits
• Adarza kavat's Cat's eye ability works on mechs. That is crazy
• [Fluctus] has infinite punch through, but the projectile lasts for about 53 meters. The Orphix's Resonators have object health like crates and containers so you can't crit them
• The Ironbride build provided is MAXIMUM damage to Orphix, since they are sentients -> status immune, so just biggest damage multipliers (radiation, cold, CRIT)

What to do:
1. Learn the Orphix's FIXED spawn order by experience or from the Wiki
2. Cast Jade's 2nd ability -> switch to weapon damage buff -> park her oustide the orphix bubble -> mech now has tons of damage
3. Kill the Orphix's resonators with the Fluctus (remember to stay in a 53 meter range for the shots to connect) -> Orphix is now open
4. Use Bonewidow's 4th Ironbride -> Heavy attack Orphix until dead/closes at half health
5. Survive with [Bonewidow]'s 1st ability (and maybe 2nd, but only if full on energy and health)

Here's a vid:
https://www.youtube.com/watch?v=LvehKm5Z0io