Unfortunately, it’ll take a lot of grinding to farm Nth metal, which is required to upgrade your various suits and weapons. The only way to obtain more of this item is to defeat enemies.

Every enemy you face in Gotham City will have a chance of dropping Nth metal when you defeat them — but not every enemy will drop the resource, as its drop rate isn’t particularly high. Tougher enemies have a much higher chance of giving you Nth metal when defeated, but these will, of course, be harder to beat.

Source: https://www.distractify.com/p/how-to-get-nth-metal-gotham-knights