In reference to the patent: US20150118365
By providing a third party observation to the USPTO. This is basically sending prior art and reasoning to why the patent application should not be granted to the examiner. They should carefully consider this, however you will not be part of the examination process, so for example they won't ask for clarification.
You can easily do this through the EPO website, just go to the patent application and look for the submit observation button.
First check patent is granted or not by US patent office. You need to file post grant opposition for above patent if it gets grant in future.