Onslaught is one of the few ways to increase firing speed, which is normally 1 second, and you can literally mow down your enemies with fast movement. I will explain the basic build I use with great success, and go over 3 key points on using this mastery.

Obligatory link to sylph's guide for basics.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2714349654

2. Hellrunner.
3. Hellrunner.
4. Hellrunner
5. Sustained Fire.
6. Onslaught.
7. Furious.
8. Running.
9. Running.
10. Onslaught.
11. Running.
12. Tough as Nails.
13. Tough as Nails.
14. Onslaught.
15. Tough as Nails.

You need Running to take full advantage of Onslaught because with this mastery, you are increasing DPS through faster movement speed. If an attack normally takes 1 second, you can fire 3 times in 1 second with Onslaught since Hellrunner level 3 and Running cuts movement and attack to 0.35 seconds. Running will also negative the dodge penalties with Onslaught level 1 and 2.

From my experience, archwarlocks are the biggest threats to you in the endgame so now I been grabbing Tough as Nails level 3 to negative the pain penalty to aim. If you are pain immune from warlock's eye or the biolabs reward, you can take sustain fire level 2 and 3.

NOTE: 1.7 change to JS armor means that SoG3 will not guarantee 100% crit so I would grab sustained fire at level 16 now especially considering the change to critical damage with rotaries.

For Callisto, any branch works and it is up your preference. My personal preference is Mimir because of the Mk2 manufacturing station that will allow you craft a AV2 basic armor for a possible chance at the loading-feed perk.

For Europa and Io, try to go for a branch with the AV3 AMP for the most used weapon group (Europa Ruins, Refueling Base/The Pit, Black Site, Shadow halls) for the flat damage boost to your rifle/chaingun.

I have seen comments where players are resistant to taking Onslaught as early as possible because they fear the dodge penalty. I will tell you just take it at level 6 and thank me later. The biggest problem you will have in Europa is getting swarmed by reavers and kerberos and normally, you would retreat to put some distance between you then fire. However with Onslaught, you can kill the reavers and cryobears, who are much more of a threat close-up, by shooting while retreating. I do find that I do struggle in Europa when I am trying builds that ramp-up slowly because of the reavers and kerberos and Onslaught makes a big difference enough for me that I will rush it at level 6 everytime. Against ranged enemies like formers and ravagers, you can still stay in cover and shoot. Even if you do need to retreat, the first movement back will likely kill any former. Unless you have running, you likely would need to stand your ground against ravagers.

I seen some recommendations to take Army Surplus and I do not agree with this. Skill points are precious and it is a much higher priority to get Hellrunner and Running as soon as possible. You should never be running out of 9mm ammo in Callisto. Now it is possible that you might be short on 7.62 in Europa, but I would advise that you be proactive and start hoarding 7.62 ammo from Callisto to prepare you for this need. In Io, there might not be enough plasma ammo initially so don't throw away your 7.62 rifle but keep it as a backup.

Most of the time, you will not be fortunate enough to find an auto-loader weapon or an armor with loading-feed and the reload time for a rifle is 1.5 seconds, which is enough for some enemies to double move against you. Speed-loader, with its 75% loading time decrease, makes reloading a rifle now 0.375 seconds, which means that you can reload and fire before an enemy can potentially act. While this only applies when your gun is empty, that is the exact movement where you need the faster reload. This is the first mod you need to add for your rifles. Thankfully, there are guaranteed tech stations at Europa 2 and Io 1. At Europa 2, you should have enough multitools to mod speed-loader, calibrate 1, and auto-calibrate your 7.62 assault rifle. At Io 1, just add speed-loader and auto-calibrate the guaranteed plasma rifle drop with optimal range of 5 at base.

When reloading, try to manipulate yourself so that when you are about to run empty is the moment you can break line of sight so that you can take cover, reload, shoot then start running again.

Personally, I prefer rifles over chainguns because of ammo efficiency and reloading a chaingun only gives you 15 ammo. Also auto-calibrating a chain gun is not ideal because you only add 1 damage point. If you have to use a chaingun, try to find those with auto-loading or efficiency perks.

Both Avalanche and Apocalypse are excellent weapons.

Nail guns can be excellent backup weapons as long as you don't have auto-loader or loading feed.

UV Angel of Confidence run to mainly show that you don't need Army Surplus.
https://youtu.be/-mbga9Yg-7U
N! example clear.
https://youtu.be/advx6Q28VOg

Thanks to CotonouB's post about Onslaught, which really opened my eyes.

https://www.reddit.com/r/JupiterHell/comments/qhmu4x/yavp_endless_diamond_onslaught_is_broken/