So I know many people consider sentinels completely worthless and while not great I can confidently say that they do serve some useful rolls not fulfilled by kubrows, kavats or their infested counterparts. This guide is mostly for newer players but there may be some info in here that proves useful to older players too.
What kills sentinels
There’s a common assumption that sentinels will die to any stray shot and while this is true stray shots are not actually a frequent cause of sentinel destruction, by default sentinels have a smaller hitbox and a lower threat rating than warframes; with most enemies also being slightly taller than warframes an unprovoked enemy will never be shooting at a sentinel (excepting bombards and everything in orb vallis)
What does kill sentinels is not-stray shots, any attack and some precepts will raise a sentinels threat rating towards the affected enemy usually resulting in rapid destruction of the sentinel.
How to keep your sentinel alive
There are two main options to keeping your sentinel alive, both are fairly basic but only the first and more limiting option seems to be common practice.
Don’t give your sentinel a gun, this is the most obvious and well known option however it relegates sentinels to nothing more than stat-sticks or GiveMeTheDamnPlant/Fish/Scan bots.
Ensure that any time your sentinel shoots an enemy that enemy WILL be inflicted with at least one rad proc, this is a far more effective method of sentinel surviveability as rad procced enemies will not consider the sentinel to have a higher threat rating treating it as if it had never hit them in the first place while still allowing the sentinel to actually contribute to combat. Additionally rad procced enemies can draw aggro from surrounding enemies reducing the volume of fire aimed in your sentinels general direction.
Uses for sentinels
This is something of an informal personal tier list and will not include all sentinels but rather the ones I find most useful.
Dethcube: there are two main uses for this sentinel.
The first is based around the Energy generator mod which is obtainable from Cephalon Simaris for 75k standing and will cause the sentinel to drop an energy orb every 10 kill assists, for the purposes of the mod a kill assist is simply dealing even a single point of damage to an enemy; For this build I recommend a Helstrum which is featured in the weapons section of this guide.
The second use for Dethcube is as additional firepower for cloaked frames around mid level (50-60) as like many companion precepts the Vaporize mod can activate even when the sentinel is cloaked, the cooldown is still 10 seconds however the attack scales with equipped weapon mods making (prisma)burst laser ideal for this build due to the power of secondary weapon mods.
Helios: potentially even more versatile than Dethcube however not quite as well scaling there are three main uses for Helios.
One notable use for Helios is as one of the few sentinels actually capable of a meaningful damage output with Deconstructor competing for most powerful sentinel weapon, this is an excellent damage dealer around mid level (50-60) and potentially beyond however for endurance runs or steel path the fall off is sharp and its often better to simply use a Helstrum for status utility purposes.
Another important use for Helios I was surprised to learn isn’t universally known is as a stat-stick, gladiator mods equipped on Deconstructor will apply to your melee/exalted-melee weapon, however they will not stack with themselves if also equipped on your regular melee.
Finally Helios works as a GiveMeTheDamnScan bot through the use of the Investigator mod.
Djin: while nothing special in regards to precepts Djinn is the only truly immortal sentinel via the Reawaken mod and thus ideal if you want a sentinel soely for the purposes of utility status application; additionally if you have or can obtain a copy of the Thumper mod it has double the attack range of most other sentinels.
Wyrm: Negate is another precept mod that some people miss due to being hidden behind Simaris rep and can be particularly useful to frames that are otherwise vulnerable to status (especially toxin) effects; otherwise Wyrm has no major uses besides fashion frame.
Oxylus: A niche GiveMeTheDamnPlant/Fish bot but still a useful niche for those nightwave silver grove specter elite weeklies or saving on luminous dye.
Making the best of sentinel weapons
Much like the previous section this is something of an informal tier list and only includes those that I find most useful.
Helstrum: Build for viral/rad status, multishot and fire-rate with (primed)firestorm if you have it (ignore damage entierly) and this is the premier status spreading sentinel weapon able to coat entire groups in radiation and viral status; ideal for condition overload or an energy generator dethcube build due to the sheer number of enemies hit with each volley and the safety assuring rad procs that are absent when using sweeper or a gas Cryotra.
Deconstructor: Essentially covered in the Dethcube section this weapon deals excellent damage for a sentinel weapon and functions as a gladiator mod stat-stick.
Burst Laser: A terrible weapon in and off itself however an excellent statstick for Vaporize or the Hard Engage moa precept.
Verglass: Surpassing Deconstructor for most powerful sentinel weapon build it for either viral/rad or viral/heat if your feeling risky, the only reason this is at the bottom of the list rather than the top is a widespread and fairly well supported speculation that the weapon is going to be heavily nerfed sometime in the near future.
Its true that sentinels are not the best companions however they are far from useless and when built right can be taken on long survival and/or steel path runs without having to gimp their effectiveness in exchange for surviveability. I hope you found at least some of this guide helpful and that some who read it may give sentinels another chance.