I believe this challenge is impossible to obtain. The tracker on the achievements page does not update and will always remain at 0%. Hoping this was just not being tracked on the page but still is possible to achieve I have been playing bf4 for quite a while now. I did not get the challenge. Also no tracked gamers on this site have the challenge. So i am assuming it is impossible. I am going to continue playing until time is up and hope I get it. Otherwise If you are planning on playing online just for the challenge you might as well not waste your time.
EDIT: They seem to have fixed the issue. The issue was fixed with approximately 12 hours remaining on the clock. It is still currently possible to get this achievement. It seems that when playing defuse it actually sometimes counts more rounds than it should. I can confirm that this is happening and if you play defuse for about 2-3 hours than you should have enough rounds and the challenge will pop. So depending on when you read this, it's not too late! Thanks to all the people who commented for this information.
EDIT EDIT: Just another quick update on this challenge. As you can see in the description it now gives you 21 days to get the 100 rounds (it used to be 14 for people out of the loop), so you now have plenty of time to gain the challenge. As said above defuse is the fastest way to get your rounds in.