Lord Uther Medicae - Support - Stimm Supply
 Hive Scum Build - Warhammer 40k: Darktide 

At the start of the match; use your Cartel Special Stimm, then immediately followed by dropping your Stimm Supply.

The moment your Stimm Supply is available to use again, pop it.
Shortly thereafter, your Cartel Special Stimm should refresh, allowing you to use your Stimm Supply ability again, because of the Practiced Deployment talent.

Go ahead and use that Stimm Supply in a more "timed manner" where a large group of enemies are, so that the supply will explode, and buff your teammates fighting in melee.
Now you should be at 4 stacks of Dependency, of which maintaining should be far easier.

When using the Needle Pistol, I recommend "Blue Mode" for Hordes, and "Yellow Mode" for Bosses & Elites.
Before fighing a Horde in melee, I will often pump a clip of Blue rounds into the Horde, then I drop the Stimm Supply, and start hackin'
In situations where there are Gunners/Snipers/Ranged Units, Elites or Normal, I will sprint towards them with a load clip, kiting all the melee enemies along the way. Pumping shots into them as I go, switching between Blue and Yellow, as needed.
At around 50% Stamina, or when I need to, I stop to reload, and run back through the Ranged Units, towards my teammates to maintain Coherency.

Upon running out of Ammo, I swap to my Heavy Sword Mk VII, and try to pick off an Elite or Specialist, causing your Pickpocket talent to restore around 25 rounds of Ammo.
Note, this can be very difficult with a DoT/Support Build, even with Hyper-Critical and an +50% critical strike chance, if your teammates are Kill-Steal Hungry. In those situations, I recommend to take advantage of when they are pinned by Plague Hounds, and use the sword to get the Plague Hound off of your teammates.

When using the Heavy Sword, I recommend using a mixture between Light Attacks, Heavy Attacks, and the Special Attacks, in a very improvised way akin to playing a guitar on the fly.
Go by whatever feels right in that moment.
The Special Attack applies a 10% Brittleness debuff, and does more Weak Spot Critical Hit Damage, so it can be utilized for taking out armored units when they are low on health with your Hyper-Critical talent.

Even when out of Ammo, if you are being shot by a bunch of gunners and fighting off a Horde, pull out your Needle Pistol, and sprint towards the Ranged Units to take them out.
This scenario is especially useful when you are dealing with Snipers, as you will be immune to their snipes, and once you reach them, you can get about 25 rounds of ammo via your Pickpocket talent.

During Boss Fights, do whatever you can to get some Ammo if you don't have any, then pump a clip of Yellow Rounds into the Boss, and buff your teammates with a Stimm Supply if possible.
Then, after a clip is dumped into the Boss, swap to Blue Rounds, and start pumping needles into the Horde that is usually present, using at least 1-2 rounds of every clip on the Boss as well, to keep the stacks of Chem at 30. The Boss usually goes down extremely fast.

I recommend using Stimm Supply the moment is available, and saving the refresh from your Cartel Special (Stimm Supply) - for dumping on the nearest Horde of Enemies, or when your squad is grouped up.
This will allow you to maintain 4 Stacks of Dependency at nearly all times.