There are plenty chances to destroy 30 tanks without the need of wasting your coins.
1.Operation 2 Mission 4 : You drive in a vehicle and have a few rockets - You cant miss the tank here.
2.Operation 3 Mission 2 : There is a Tank right at the start of the mission . When hugging the hill to your left you will find a medal , some tags and a rocket launcher.
It doesnt matter if u suicide or get killed, the progress towards the achievement saves as soon as you get the kill :)
Edit: You can abort the mission after you blow the tank up so you keep your high rank troops and the kill still counts. Thanks to "CanadianWhizKid" for the info !!