This method is by far the most efficient, despite what the others may tell you. Doing this I was able to manage 21 evasions per match, assuming I got the timing right.
First and foremost you'll want to do these achievements last, simply because there's a chance you'll obtain some via story and hero mode respectively. This isn't required, however and can be done fairly early in the game. If you have any skills from hero mode that bolster health or any kind of defense, I suggest you use them. I used Health +1 and Kongo, though for this it didn't matter too much. Here's how I set it up....
1 player match
1P vs 2P
P1 Super Saiyan 4 Gogeta (I don't think this matters much)
P2 Kid Gohan (The only part that matters about this is his energy blast barrage super).
Alright! First and foremost is your method of bolstering the spirit gauge. At first, I thought strong attacks were the fastest way like many of the solutions above but that's not the case. Thanks to admeister over on ps3trophies.org, I learned that charged Ki blasts (hold triangle) actually charge one whole bar per use. This means that after four charged Ki blasts, my second controller was clear to use a super and my main only took 2400 damage. While firing blasts with the second controller I would hold down on the d-pad with P1 in order to boost my ki, especially since each hit drains you a bit. Once your spirit meter and Ki are where they should be, use the super by flicking up on the right stick.
For evasion, you'll need around half a bar in order to use the defense. It'll prompt you once the super is heading your way, so be ready for that. Once you choose evasion, it'll bring up a bar at the bottom of the screen with glowing lines sliding across the screen. Thankfully, you only have to hit Square when it's past the blue bar and it doesn't have to be dead center. Because of this, you don't have to be quite as fast as you think. If you manage to dodge them all, you'll be able to get 21 per match as I said above.
If you think this solution could use anything extra, feel free to comment!