Septian is a fantastically fun champion that has a lot of innate lifesteal built into his kit. Although he may not offer a lot of initiating ability besides his ultimate, once Septian is in a fight, it is very difficult to kill him. However, although this guide may bring you success with the character Septian, it is not guaranteed that you will win all your games. Games are won by objective control and teamwork, not solo kills and legendary weapons.

Details
Getting the second augment on R as soon as you can is pretty important for Septian. This allows him to deal a steady stream of DPS (damage per second) in the middle of the fighting while allowing himself to life steal and take reduced damage. In essence, he does a lot of damage while taking (40%) less.

For the next skill to max, it really depends on your personal preference. Personally, I like the second augment on Q first because it allows me to shuffle my opponent into the fight or back into a vulnerable position. However, I can imagine someone getting second augment on E first to catch escaped kills if the opponent is very mobile.

This build aims to stack Life Leech, Attack Speed, and Critical Strike to maximize damage and survivability at the same time thanks to Septian's innate life steal.

The Weapons

Three Point

Tier 16; Legendary
540 power
70 critical strike
40 attack speed
28 life leech
# 33% Critical Toxin

or

Acid Injectors

Acid Injectors

Tier 16; Legendary
540 power
62 life leech
62 attack speed
14 critical strike
# 25% Blight

Mods:
Critical Advantage
Gladiator
Charge: Life Leech or Critical Strikes

Both of these weapons have all the stats we want. The secondary attack gives Septian a means to stick onto opponents or escape from a fight gone bad. The passives just add helpful damage. Personally, I highly prefer Three Point much more due to higher Critical Strike and higher Attack Speed. When these two stats are combined with the legendary mods Critical Advantage and Lethal Strikes, you will make up for the Life Leech that is lost from not using Acid Injectors. CRITICAL ADVANTAGE IS SUPER IMPORTANT. It gives you a crazy amount of life steal on your crits, which you will be doing a lot of with this build.

12-Gauge Grass

Tier 16; Legendary
540 power
62 life leech
48 attack speed
28 critical strike
# 20% Leeching Acid

Mods:
Lethal Strikes
Initiator
Charge: Attack Speed

This weapon is amazing for Septian. It has all the stats he needs for this build, and the passive is really nice when you want to step back from a fight and do some ranged damage while healing up.
If you don't have this gun, here are some replacements:

The Conductor: This would be my first pick if I didn't have 12-Gauge Grass. It's passive is excellent for chasing and initiating. With high Critical Strike, it is a great second choice.

The T-Wrecks: It gives a great amount of Critical Strike and some Attack Speed. However, it doesn't have Life Leech.

Bleach Sprayer: It's got what Septian needs: Critical Strike, Attack Speed, and Life Leech.

No Aegis or Survival Adaptation Legendary Mods?

Meh, these mods aren't necessary when you life steal more health than your opponent deals (if you're not outnumbered).

Gadgets
You want Critical Strike and Life Leech.

You want to pop your R before you enter a fight. Hopefully, this fight has been started already by your teammates or by the other two teams fighting. Try and use your Q to separate one enemy out of the fight and kill them with your crits and attack speed. Your Envenom passive will weaken them as you beat them up.

You can solo Looters once you have the second augment on R and you have cleared surrounding areas for Elites that might disrupt you. I don't suggest this unless you know the other teams are occupied and your teammates don't need you.

This guide will be updated more as I continue to explore ways to play Septian. Meanwhile, enjoy this build and I hope you can win some games :)