The first thing you’ll need to do is complete the Tidal Basin Stronghold mission at least once, then do a couple of Invader missions and complete the Capital Building Stronghold mission. After that, you’ll need to return to Tidal Basin.
Playthrough the Tidal Basin Stronghold mission again until you get to the Acquire the Antiviral objective. There’s a building on the right of the gate you’ll need to enter once you’ve cleared the area. Go up the stairs and on one of the bunk beds will be a Black Tusk Keycard.
Play through as normal until you reach the point towards the end of the level where you need to board the hovercraft. Once you’re in the room where you’d turn right to progress through the Stronghold mission, hug the wall to the left to find a door with a keypad.
You can open this door using the Black Tusk Keycard you found earlier. Inside you’ll find a weapons crate that contains a guaranteed drop, the Adrestia SR1 Sniper Rifle. Once you’ve obtained it, you’ll need to dismantle it if you want to get the Nemesis.
When you dismantle the Adrestia SR1, you’ll get The Tally Marksman Rifle Scope, which is the first component you need to craft the Nemesis. You’ll see in the description the words Klutz, Shorty and prime, which just so happen to be the nicknames of the Invaded bosses.
You’ll need to complete each of the Invaded Strongholds on Invaded difficulty, but only the Capital Building Stronghold is available this week. They rotate weekly, so you’ll need to wait another two weeks to get the other components.
Head to the Capital Building Stronghold and toggle Invasion and defeat the Boss, Prime, to get the next component, The Scourge Marksman Rifle Barrel.
It’s not clear whether this is a guaranteed drop or not, but it’s definitely your best bet of finding it. But do what you want – I’m not your mum.
The other two Bosses are in the District Union Arena Stronghold and Roosevelt Island Stronghold, so jump in when Invaded difficulty is unlocked.