BlazBlue: Continuum Shift

Easily done with two controllers. In local versus, pick Tsubaki for your character and Tager for the second controller. Once the match starts, get as close to Tager as possible, then start walking towards him and executing Tsubaki's 6C attack. Immediately after you hit the button on Tsubaki's controller, hit the drive button on Tager's and it should count as a fatal counter. Got in less than five minutes with this method.

The solution that worked best for me is the following:
"the easy way to get your 30 is by picking Bang and having the opponent be Jin. Have Jin start his 5D attack and then quickly use 3C before the attack hits you. This will result in a Fatal Counter, so simply repeat 30 times"
Hope it helps, I was playing on my own with two controllers and you just need to manage to press C for Jin, Diagonal down+forward (3 on the keypad) + B.
3 rounds tops, achievement unlocked

A "Fatal Counter" occurs when you land an attack with the Fatal property as a counter move. Moves with the Fatal property are usually guard breaking moves as well. Each character has one or two that will count as one.

I got this one rather easily as Tsubaki, just doing her arcade/story mode. Her 6CC attack (which is Forward + BB on default controls) where she attacks with a downward strike and then knocks the enemy into the air. It's pretty powerful so you'll use this attack as part of her moveset as she can then combo into the air very easily. Just keep up with it and it'll eventually unlock.