Expand the Parent Layer, confusingly called a "Layer Layer" and scroll down to the bottom, select the last nested layer and delete it, this is the contour I made, so we are starting afresh.
Scroll back to the top of the layer stack, select the Parent Layer and duplicate it, this preserves the original should mistakes be made, make sure you lock it.
Working on the duplicate, expand it and select all of the child layers and use Boolean Add, if everything goes black just go to the colour tab and flip the colours from fill to stroke
From here you can use the Contour tool to pull out an edge, don't forget to Bake Appearance so that you can edit the curve and remove edits like the ones shown in the image below.
Good luck.
