Go to the events tab (half way down on the right hand edge of the screen) and you'll find a list of this weeks events which refresh on Sunday night / Monday morning.
Most of the event pages just highlight which troop type and Kingdom are getting a 25% uplift in their stats for the week but keep scrolling down and you'll get to the one important screen - the actual event.
It usually takes the form of kill enemies from a specific kingdom to earn points. Earn enough points over the week and you'll pass milestones earning you keys, gems and new troops. Keep earning and you'll eventually earn enough points to complete the event and unlock this achievement. It will probably take upwards of 100 battles.
For example this weeks event (starting 27th March 2017) was called "Special Snowflake" and you needed to defeat 400 Glacial Peaks Enemies. This was easiest to achieve by completing the last Glacial Peaks challenge with 4 glacial peak enemies 100 times.