Gather 25,000 total salvage.

Here's a way to boost your salvage endlessly:

When you reach the point where you have to "Find your way out of the sewers" there's an area with both an upgrade and a weapons station. But most important a few steps ahead up a stairway there are a few square lamps at the ceiling.

Shooting them earns you salvage. Repairing the lamps and destroying them again gives you salvage again. You can repeat this as often as you wish. Doing so starts a rain of salvage giving you unlimited salvage plus the weapons station lets you refresh your ammo as often as you want. Don't use explosives though - they will permanently destroy the lamps (thanks to Tarzakul for the info).

Easy way to gather 25.000 salvage to unlock the achievement and to upgrade you character's abilities.