The Lost Precursor City can be located from Rock Village. Locate the dock near the weeping warrior, and use the floating timber to jump to the large Precursor building in the water. The City is almost entirely under water. There are four main enemies here. The first is your average purple Lurker. The second is actually two Lurkers, one on top of the other. You have to jump and spin kick to break them apart, then take each out separately. The third is a flying blue Lurker that expands and becomes riddled with spines. You have to hit this one when it's spines are withdrawn. The fourth and final enemy is a blue Lurker in a Spinning spiked contraption. You have to jump and dive attack on it to defeat it. The Eco found here consists of raw Blue and Green Eco, as well as Blue Eco vents.