The Division 2's update that added Tidal Basin also added Nemesis marksman rifle. Unfortunately, it cannot be acquired yet due to time gating, but we have made this guide so you can get the necessary parts as they become available.
Nemesis is a Black Tusk-themed exotic weapon and as such, it is not much of a surprise it comes from invaded Strongholds. The first invaded stronghold you will need to (re)visit is Tidal Basin.
For some reason, Massive and Ubisoft thought it would be nice to send players on a replay of Tidal Basin just after they complete it. Completion of this stronghold is required to move from World Tier 4 to World Tier 5 and the first part you need for Nemesis can only be acquired in the latter version.
Therefore, you will need to mostly complete it in World Tier 5 as well. So, go ahead and complete all the invaded missions and a stronghold that's on the rotation before getting into Tidal Basin.
After finally getting into the Black Tusk base, proceed until you get the objective "Intercept Black Tusk comms". Head on over to the building the objective is located.
Instead of going for the console objective points you to, take the stairs next to it and go to the second floor that contains bunk beds.
You will find Black Tusk Keycard next to one bed - pick it up.
Continue the mission until you get into the hovercraft. You will be mostly through the section by the time you get to use the keycard as the corresponding door is located in the control room with three big blue displays, one of which shows the Warhound blueprints. Face the wall opposite of where the "Reach the upper deck" objective leads you and you will be able to use the keycard.
Inside this room, you will find a weapon crate that will yield Adrestia SR-1 unique High-End marksman rifle. As odd as this may sound - salvage the weapon for the first component which is the scope named The Tally. Upon examining this item, you will see the hints referencing three Black Tusk bosses who hold more parts of the weapon - Klutz, Shorty and Prime.
One of each of these bosses can be found in invaded strongholds. Capitol Building has David "Prime" Maxwell, who will drop the barrel called The Scourge.
Roosevelt Island invaded stronghold will have Erin "Shorty" Linwood who will drop the marksman rifle bolt called The Scales, while District Union Arena boss will be Aurora "Klutz" Kay and she will drop the exotic stock.
Update: 18 April 2019 was when the final rotation happened and made Nemesis available for the first time.
The final component, the blueprint itself, will drop from the first boss in Invaded Grand Washington Hotel. The boss' name is Puck and she will appear in the hall at the beginning of the mission. Kill her, loot the blueprint, craft Nemesis and buy the upgrade blueprint from the White House vendor to upgrade the weapon to 500 Gear Score.
It is also with great pleasure that I can inform you that Nemesis is an absolute monstrosity in PvE and is therefore absolutely worth getting, despite the time gating. The talent that required a charged shot may have put some players off, but after testing it, the weapon feels great. Charging the weapon up is fairly quick and it doesn't require you to fully charge it - the damage scales based on the charge.
In case you need quicker shots, you can do so at about 33 per cent charge and still outdamage any other marksman rifle in the game even without the tracking bonus applied. Fully charging the weapon can one-shot bosses and it ruins juggernaut and Warhound armour plates even without a completed build.
Originally, World Tier 5 players could group up with those in lower tiers in order to skip the time gating and enter invaded missions and strongholds as necessary in order to get the components. We haven't tested whether Massive patched this out but it is worth a try - it could literally save you one month worth of waiting.