Here's a tip for her exalted guns. The AOE is affected by mods on both the gun (Primed Fulmination) and the frame (range mods) to make some nice nuke pistols.

IDK personally, but Blazing Pillage augment is your only way of restoring shields against Infested

The longest someone has gone is of course level cap - for infested you need to include the Blazing Pillage augment into your build since infested have no shields or armor typically, so the only way for pillage to restore shields against infested is via that augment.

I specifically avoid doing long sp infested survivals on any frame, the sheer amount of annoying ancients are far too high for my enjoyment. 2hr sp grineer survival is a breeze comparatively

Someone like Revenant completely eliminates any threats they may pose via literal immunity to all status and damage, Nezha is another good option.

Condemn is a decent helminth option to make infested shield gen easier. Steel path feels a little nicer with some health archon shards as you mainly just need a little time to react and have your next pillage clear any status. I run her with 1 health, 2 duration, 2 str shards.

The rest of the time i run elemental ward electric element -- has weird synergy with hildryn shield drain/casting counting as damage and reflecting it to everything.

Overall though infested is always a little tedious with her as even with condemn or blazing pillage you can still die instantly on infested missions.

For completeness I guess i'll add I use her for archon hunts. Do the missions in normal gear/elemenetal ward and the hunt mission use phenmor + xata's whisper.

I helminth her 4 and never use her 1.

You can only restore shields with an augment and you need an arcane to not take toxin damage because her passive won't work half the time because you will not have over shields because infested don't give you any.

Well, if you have access to shards, giving her health would be an option as usually the damage done to her in toxic form is usually +500 damage, she won't survive most toxic attacks with just 300 health.

Using wisp helmith ability sure helps her out dealing more damage, not only with weapons but with her exalted secondary, which is one of the best AOE weapons in the game as it doesn't suffer from knockback, self damage or ammo issues.
You should however not use her exalted weapon against everything, eximus and some infested units resist abilities, making the damage low or insignificant, so make sure not to exaust all your shields shooting at something that takes 20 hits to kill, use a melee or another weapon for those.

Blazing Pillage is the main bandaid for pillage's ineffectiveness against infested.

Subsume Condemn, and/or use Blazing Pillage

Hildryn is unique in that so long as she has overshields, any toxin damage will damage the overshields and not her health - the overshields even provide a shield gate if depleted, where any remaining toxin damage from the hit is negated and will not pass through her normal shields to her health. In this way, she is the only frame that can shieldgate against toxin damage, which makes her an ironically strong pick against Infested, IF she can maintain overshields

So the long story short is maintain overshields, move around lots to minimise getting hit, and make use of CC. Consider Rolling Guard for tactical immunity. Blazing Pillage doesn't quite keep up in my experience, even in high enemy density, so I always run Condemn when using her against Infested

If overshields drop and toxin one-shots her, it feels bad. But it feels no worse than getting one-shot on literally any other frame vs high level Infested