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Mod 1: Vehicle Regeneration mod from Autonoms.

Mod 2: Any mod which increases heat damage.

Mod 3: Either go with an offensive or defensive mod.


You might also want to play around with the feature that allows the walker to regenerate HP while standing in burning terrain. I don´t remember if this was a default trait or it was granted by a mod.
 
If you only consider secret tech mods, there's actually not that much choice. Backlash Projectors and Superaheated Plasma Actuators look both pretty good. PrX Absorption Panels are already part of the Walkers basic abilities (as Tropical Sunset mentioned), so useless, and Phoenix Bomb Launchers seem a bit wasted on a melee focused unit (it might still be worth it with a very defensive army composition, though). On the other hand, Hazmat Protection would be imho a good choice again, especially if you're facing a lot of enemies with arc weapons (to reduce stun chance).

Regarding weapon mods, most of them don't work but Arc Retaliation Defense and Positron Discharge Shield are two exceptions that seem strong - if you have access to arc. With Vanguard I would also consider Jetpacks and with DVAR Super-Powered Pistons.
 
Trebor, not sure if you care at all about this any more, but I wanted to shoot something out there.

And this is based a bit on my bias against the Phoenix Walker. And the fact that it doesn't have a lot going for it in PVE. Or in Empire.

But there are two giant vulnerabilities of the Phoenix Walker. It has to 1 - cross the terrain endgame, which is basically suicide, and 2 - it has a massive arc weakness, which seems like 1/2 of everything.

So... if I was to do something with this unit, assuming I got it early, I would go Assembly or Syndicate, and load it up like this:

1. Therian augmented legs
2. Arc Retaliation Defense * gives 2 arc resistance on top of damaging shields and stun
3. Purification Field

That is a cheap unit. Plus, it is faster, which allows it to take the hits and close the gap - WITHOUT being completely vulnerable while it does. When positioned, attackers have to get 3 hexes away before hitting it, or they are likely stunned. And it has a heal, which cleanses status effects. Still a bit weak, but versatile.

Then endgame:

1. Arc Retaliation Defense
2. Positron Arc Storm Projector
3. Positron Defense Shield * 6 arc resistance, closing the big arc gap. And otherwise just crazy good.

That is a tough unit. If we can assume +2 "well-built," and maybe 1 or additional armor from some landmark, we are talking now about 9 base armor, 3 base shields, and +8 arc resistance, sealing the biggest gap. Plus, it has a massive air/ground arc storm it can release on an arc channel, plus massive fire damage from its base attacks, amped up with +30% damage from mods. That is a tough unit that can maybe actually get through the killing field, and do something on the other side.

I still think the unit is 'meh,' and slow, but it might have a function.

I also REALLY like the idea of jetpacks, which might let you cross the field, though just once, and it has no additional value. The Arc shields are WAY better IMO.

Anyways, there it is.