With the dark side build, you forego anything that creates light for the incredible benefits of the top "Concealed Fatigue" skill tree. The skills slow enemies outside your vision circle to a laughable crawl, then shrink your light circle while increasing fire rate and damage to savage levels, and culminates with a +500% XP collection range that yoinks cash from nearly the entire screen. Beyond that, this build is pretty basic. But damn is it effective!

Note: this works with any weapon. I got the 3 mil achievement with the greatsword. Also, I won't discuss perks, as they are weapon and playstyle dependent, except to remind you to avoid anything that creates vision.

Early on, it's easy to get mobbed, so dealing max damage to max enemies is key. Prioritize the Bulk Rounds, Quick Draw, and Shrapnel trees. I personally like getting to Bullet Flower (even with low fire rate, because later your fire rate will be anything but low) and Calamity Rounds, but anything that is a net improvement to fire rate and damage is great. I generally avoid Death Row for its debuff to fire rate.

Now we fully embrace the darkness. Concealed Fatigue, Patient Stalker, Blind Fire, and Blind Telekinesis add a ton of power to an already solid build. The primary power of this build is that these 4 upgrades functionally replace the 7 upgrades it takes to get Light Wave and Light Magnet, on top of adding a ton of damage and fire rate. Those 3 upgrades may sound small, but considering I hit the final boss around level 32 or so, that's about 10% extra power. Factor in whatever you might have spent on the Radiation, Gleam, Flashbangs, and Spotlight trees, and you've got a ton of upgrades freed up to focus on the raw power of your build.

While you're working to make it to Blind Telekinesis, Stopping Power, Full Metal Jacket, Spray and Pray, Bullet Flower, Slug Shrapnel, and Calamity Rounds, it's time to focus a bit on your survival. I like Shield Barrier and the bottom tree of Slowed Projectiles (at least until Meet Your Maker). You can grab Bullet Cyclone, Rage, and Shield Boost, but I'd avoid the rest (since they add light, and light bad).

Assuming you picked good perks and didn't get boned by RNG, you should already be pretty busted leading up to the final boss. But it's a good idea to add damage, and if you're going for 3 million points, you will need it, as the enemies get real tanky after about 2 mil. For high fire rate weapons, I lean into bleed (all of it is really useful), and for lower fire rate weapons, I lean into lightning (I don't love the Thunderstorm branch, but the Tempest one is great). Burn, Guided Missiles, and Singularity all add small amounts of damage but I wouldn't take them unless there's nothing else. Now is also a good time to grab a little movement speed (I usually only take the first level Swiftness, but if it's all there is Magnetic Rounds and Run and Gun are decent).

The only real flaw of this build is that you don't have a reliable way to freeze enemies (some perks might help, but can't be relied upon). Slowing everything outside your tiny vision circle is great, but without a way to stunlock enemies, this build probably won't get you to infinite points.