Image: UbisoftOfficial screenshot from Ubisoft.
Armed to the Teeth asks you to collect 49 Unique Weapons, the named guns with fixed mods and their own look. It is one of the long collection trophies, so pick them up as you explore rather than all at the end.
1. Finish Treasure Hunts in each region, since many end with a Unique Weapon. 2. Spend Moneda at Lola's black market on any you do not own yet. 3. Pick up rewards from missions and side content as you go. 4. Keep going until your collection reaches 49.
Clear every Treasure Hunt in a region before moving on, so you do not cross the map twice. Check Lola's stock often, as it changes. Run Special Operations and Bandido Operations for a steady flow of Moneda.