The Division 2's newest stronghold, Tidal Basin, is fairly big so it's no surprise it also has a few sections that may be confusing to navigate through and the final boss is a bit different so here are a few tips to walk you through.
Getting to Tidal Basin is fairly straightforward - fast travel to the nearest place and follow the waypoint to get to the sinkhole that will lead you to the base's vulnerable wall in order to blow it up. It is as simple as following the waypoints the game provides.
As soon as you make the big boom boom, an alarm siren will go off and you are presented with the first firefight in Tidal Basin. Mop the Black Tusk up and you will be tasked with circumventing the autocannons. Thankfully, these don't hit nearly as hard as the ones in the original The Division so it's not hard to bypass them - the button you need to press will be to your right as you look at the cannons.
You will then run into two separate but back to back firefights which will lead to the objective "Intercept the Black Tusk comms". Keep in mind this objective will lead you to the building where you will get the first item you need for but it will only spawn if you do the stronghold on World Tier 5.
Once you interact with the console, a gate will open and the objective will change to "Find the heliport", prompting the waypoint marker to lead you to the three helicopters you will need to disable.
Needless to say, there will be another prolonged firefight during this objective but if you want to save some time, shoot the red barrels next to the helicopters instead of giving the birds a lead shower. There are such barrels next to each helicopter but it might be harder to spot them if you are doing Tidal Basin during the night.
After you've shot up everything and everyone in the heliport area, your next objective will be to reach the main gate area. This will lead you up to autocannons again and this time you will disable them for good by shooting up yellow fuse boxes that the game will conveniently mark for you. You cannot kill autocannons in any other way but they are quite slow to react when you are switching cover.
Black Tusk personnel are, however, so make sure you dispose of them before going from cover to cover. This tip is not essential in lower difficulties but Challenge can result in you being dropped if they start shooting before you reach another cover. Following the demise of autocannons and Black Tusks, you will be tasked with opening the door for a JTF person you will then need to babysit.
The JTF team leader will take up a fairly fortified position as the console is on high ground, inside a building but there are three entrances you need to be mindful of. Two of them are immediately apparent, but the third entrance is behind a desk with laptops that will obfuscate it. The desk is directly behind the JTF officer.
When you are done babysitting, proceed as the next objective instructs you. The obvious path would be through the gate you will come across, but the actual path is to the right as the gate will never open. Continue on the obvious path after that, but you will soon run into another gate that won't open. Go through the building to the left of the gate and you will be able to continue the mission.
Your next objective will be to intercept the antiviral capsules that are being transported on modified Warhounds but even though they are mobile and the objective hints you need to hurry - you actually don't. The Warhounds can't escape you, so value your health and safely dispose of Black Tusk along the way, before giving chase to robo-dogs.
After you disable the transport, pop a flare and you will be able to go after the second Warhound transport. This section is pretty straightforward, just follow the waypoints and kill any Black Tusk, followed by disabling the second cylinder carrier.
Shoot another flare and the gate will open, revealing the SHD chopper being shot down and accidentally crashing into the Black Tusk hovercraft turbine which will make sure it can't escape you. Proceed to the hovercraft, kill any Black Tusk along the way and by the time the objective changes to "Reach the upper deck" you will likely have reached the control room with three blue screens on the wall.
The objective will lead you to the door on the right but there is also a door on the left. Remember the keycard needed for Nemesis rifle you picked up earlier? That's the door you will use it on, which will reveal the weapon you need to dismantle for parts.
Anyway, head on where the waypoint leads until you reach the "No Respawn" area, which will indicate the final battle of the mission. There will be plenty of Black Tusk you will need to eliminate, but it's the usual drill. The main catch here are the rockets you will be prompted to disable mid-fight.
Each rocket will take two and a half minutes to launch and preventing them from doing so will fail the objective. Normally, this would require you to give up the cover and run around to shoot the two weak spots on each launcher in order to disable it. If you don't want to get all shot up on Challenge difficulty, it would be wise to switch your abilities prior to the battle, as Oxidizing Chem Launcher is invaluable for these objectives.
It covers an are in a gas that will damage both weak points on each missile launcher even if you don't have a clear line of sight and you never have to leave cover. Just pop it on a launcher and let the ability do its magic.
Milla "Wyvern" Radek is fairly challenging on her own as she has a grenade launcher, armour that is stronger than that on juggernauts and smaller hitbox. You will likely have to break at least two pieces of her armour in order to shoot her effectively.
Kill any Black Tusk that come along, put down their officer that wanted to blow up the White House and that's it. Mission complete.