As others have stated, you need to unlock your 4th ability in the Dominion game mode. If you're good at the game, getting this achievement will come naturally. But sitting on a point and getting an occasional kill was not usually enough for me to max renown.
So I figured out a method that for me guarenteed I was earning max renown almost every single match. Start off by going with the bulk of your team to capture the first point. A nice starting boost. Now you can sit on this point. But instead I jumped right into the action. The idea is to build up to your second renown level as quick as possible. Now here's the part that is valuable. Choose a hero that has the flash bang ability as their second level renown (I used Warden). Once you have that ability, wait until the small AI soldiers gather at the middle point (Point B on almost every map). Regardless of whether your team has the point or not, just toss your bomb into the group of enemy AI. If you wait for them to group up, you can usually hit 15 or so. Which almost instantly bumps you to level 3. And unless one team is getting slaughtered, you will have enough time to easily repeat this step 3-4 times. Chances are you will achieve max renown after throwing your second bomb.
Using this method, I was able to achieve max renown even on games that my team lost by a significant margin. I think I'm a fairly good player at this game. But by no means a pro. So even if you feel you're not elite, using this method should make this achievement a cakewalk