How do I scale an object in only one direction?
Set the Anchor Point to the bottom of the object by pressing y or clicking on 
Disable proportional scaling 
Optional you can add a slider control to to get a better control of direction scale values




The shape can also used as track matte, just set the layer below to Alpha. 

Note: As alternative you could use a trim path on a shape layer as track matte, see this answer: 


https://graphicdesign.stackexchange.com/questions/49643/how-to-use-after-effects-trim-path-with-an-illustrator-path/49975#49975