I just won this and here are the steps to win it.
1. From the main menu go to multi player
2. On the right hand side there is a section for special events which at the moment of writing is this "Under Control" event
3. Join a game
4. Get some stuns
The game type has 3 sectors which teams compete for in a king of the hill style battle. For stuns to count they must be within one of the sectors whilst the enemy has control of it. The section must be highlighted red when you get the stun.
I found the best way to achieve this was to hide somewhere in a zone you control and let an enemy come in and take control. They are usually easily spotted because they're running in. Wait for the control to go to the other team and go to the player you saw and press X to stun. You must be quick because if two players from opposite teams are in the same sector it will flick back and forth between control by the enemy and your team. You have around 5 seconds.
Note: Contested stuns do not count, it must be a clean stun.