:Mastery Farming:
This build will work fantastic for any stealth mission but the main reason I made this was for solo leveling weapons very fast. On a good run I can level a melee and either a primary or secondary weapon both from 0-30 in a single 15 minute(ish) solo run through a Grineer Exterminate or Sabotage mission (Grineer give noticeably more XP than any other faction level for level). Go to Adaro ideally but for weaker weapons Telesto on Saturn. Enceladus is probably going to be your best bet for weaker weapons (especially secondary weapons) since the enemies are just low enough to one-shot with only 2-3 mods in the weapon, there are no Nox enemies, the enemies are spread out more making it easier to headshot them one at a time, and unlike some other sabotage missions, all the enemies don't become alerted once you finish the objective (HOWEVER.... the Lotus does like to fuck it all up sometimes by saying "Ignore your original objective. Leave nothing alive" which turns it into an extermination mission and all enemies will be alerted for the rest of the level). For some of the really weak secondary weapons, you will even need to drop down to level 10-15 enemies to one-shot them. All of this makes it super simple to keep a 500% stealth kill boost the whole time. Even at Enceladus, you can easily fully level a melee weapon and partly a primary or secondary. If you went in with the weapon at rank 0, as soon as it hits rank 20 swap to the next weapon and the bonus XP will carry it the rest of the way to 30 (assuming you're running an affinity booster as you should be).

All you do is load in, hit 3 for perma-invis, tap shift to get around faster by rolling and staying invisible while spamming Rest and [Rage] at any group of enemies. Having both invis and R&R together reduces the chances of any enemies getting alerted as much as possible. No more having to wait for other enemies to look away before stealth killing another enemy in the room. No more having to kill all of them at once with a slam and hope you get them all. No more being seen if they wake up, or before you can put them all to sleep. This eliminates both issues you might run into with a normal [Ivara] or [Equinox] stealth kill build and just speeds everything up while allowing you to be as lazy as possible in solo mode and watch Netflix or whatever on the other monitor while grinding mastery.

For obvious reasons this is only good for leveling weapons and not frames, however, the same thought process can be used to level frames very quickly also. Just slap Rest and [Rage] over their 1st skill on like the 3rd loadout, add some efficiency, duration, and range, and off you go! Keep in mind though that Rest and [Rage] has practically no AOE around the target until a certain rank, and you have to be much more precise with aiming it at the target during the low ranks. Thankfully with the new forma/mastery changes, this is less of an issue than it used to be.

Also no more Hydron AFKers or dealing with people who leave at wave 5 thinking they are being efficient because they don't understand how XP works, or all the Vaubans that show up to ESO Expecting to gather everything up but not kill anything themselves so they can leech all the xp, which is efficient, but also a dick move since that is why everyone else is there too to rank up those pesky weak weapons much faster and without having to actually try killing anything with them, so you end up with a lot of parties with multiple Vaubans, and everyone just standing around waiting for someone else to take one for the team and kill the cluster of enemies only gaining them xp on their frame (which already has to be level 30 to even be there anyway) and the weapon they used to kill the enemies with while the rest of the party gets that xp distributed across everything they have equipped. I'm not against this, but out of courtesy, everyone should switch off killing the cluster created by the [Vauban] in an ideal run where everyone leaves happy. You could say this build is the product of my saltiness of just having a lot of bad luck with parties in both Hydron and ESO not wanting to scratch each other's back.

:Focus Farming:
For focus farming get a nice strong melee weapon that will be able to kill the enemies without waking them up (The [Pennant] is a favorite of mine) and slap a Eidolon or Lua lense for whatever focus school you want to work on in it. Then if you have Steel Path unlocked simply go to Mariana on Earth (Grineer level 55-75), put everything to sleep and try to have a large cluster of enemies sleeping when you grab the yellow glowing focus thing, and then proceed to mow down as many enemies as possible before times runs out while making sure to keep your 500% affinity going. This level is also great for its layout for this purpose being almost completely linear halls making things as easy as possible. If you do not have Steel Path unlocked yet feel free to go to Adaro on Sedna, but the enemies there will only be 32-36 and the layout of the level will be much less favorable, however, you also don't need nearly as hard-hitting of a weapon either since pretty much any decently built weapon will one-shot the enemies there with ease.

:Lich's & Sisters:
I might also mention [Ivara]'s invisibility makes killing Lichs and Sisters a walk in the park. No more jumping and teleporting around and having to chase them down, they just casually walk around like "What's shooting me in the face I wonder quietly to myself?" until they are dead and you have a shiny new toy. Also for Lichs you can use the noise arrow to try to make him spawn more thralls faster.

:Cephalon Simmaris:
This build is also PERFECT for grinding Simmaris standing since you get triple standing for a stealth scan and you can put things to sleep and uh....un-alert them after a few seconds if needed. Preferably go to a Steel Path mission and you can max out your daily standing easily in one run.

:Animal Hunts:
This lets you cheese animal hunts as easy as you're going to get. Just stay invisible, then once you use the echo lure point towards the creature and use Rest and [Rage]. No more actually having to hit the target with a slow moving dart, works on flying targets just as easily as land targets, and will put a cluster to sleep if there is more than one. This is the easiest way to get Common Avichaea to progress on Demos other than waiting for the tag to come into Son's rotation to buy with plat.

:Mod information:

You could use [Constitution] for an unnoticeable increase in sleep time, but having the overshields is a nice safety net just in case.

You can run pretty much any Aura since Energy Siphon is minuscule and redundant anyway, and Corrosive Projection might give you that little extra to one-shot a little higher level enemies with that much weaker of a weapon.

This is soooo much faster and easier than using the sleep arrows. Much larger AOE, you can spam it, and they stay asleep much longer.

[Infiltrate] for a little faster movement and to ignore those pesky sensor doors that drain your energy.

Arcane Energize is a must, but the other one can be whatever you have that you think will help. Arcane Precision would probably be a big help when leveling secondary weapons.

It's not a big deal if you use a Rank 3, 4, or 5 Fleeting Expertise. You won't notice a difference, Rank 3 would just be the most min/maxed.

:Bonus Information:
If you get tired of doing this just do railjack Skirmish missions (void storms are even better for the added loot) and you get a nice chunk of XP for every hacked console you do in those, so board the crewships and take them out the old fashioned way, and also any consoles in the main objective area. It's not nearly as fast as this, but a good way to passively rank up stuff you don't want to use without relying on other people, and you make progress in other areas of the game as well like this.