Image: UbisoftOfficial screenshot from Ubisoft.
Oh No You Don't! asks you to take out 3 Insurgent Leaders. Insurgencies are a weekly endgame mode that opens after the main story, which is why only 1.3% of Steam players have it.
1. Finish the main story. 2. Start an Insurgency. 3. Clear objectives to reveal the leader. 4. Take them out. 5. Repeat until three are down.
Each leader drops a unique reward, which also helps Armed to the Teeth. Since they reset every week, check back each week until you have three.