Battle Group 2

Unfortunately, kills done with Iowa (cannon kills) do not count towards this achievement, so you need to use missile-launching ships to grind this.

A fast way to grind is to play level 25 (the one before the final area) with Arsenal and a whole lot of Hyper Times. That level has 7-8 missile launching planes (the ones that look like passenger planes) in total, and most of the time the enemies come in big groups, so you can get many hits per hyper time. I was getting around 200 hits per game in exchange of 5 hyper times.

(If you lack hyper time, do a complete playthrough with Iowa and the aircraft carrier. You get around 120 oil barrels per level assuming 100% accuracy, so you can easily stack hyper time.)