Dragon Ball: Raging Blast

still didnt unlock for me this way

I can confirm this method works. If you didn't get the achievement this way, then you've missed a character. Just like I did, but I went back through a few of them until I got it. Also to clarify I did it without completing a single match. As soon as each fight started I went straight back to the character select screen.

Thanks for the unlockable list.
I just want to add a side note which is probably obvious to the most: at the end menu screen when you finish the battle, in case you're actually fighting, you can pick "Go to character selection screen". Eventhough it says "sure? all the non saved progresses will be lost", it doesn't mean anything. It does save when the match ends before the menu to proceed appears, and that's the save you need. So you can go at the character selection screen again. I did it, and it worked. This might be useless, again, but I read some threads on other forums with people having issues at unlocking this achievement, and some of them were wondering if this save system worked correctly when you go to the character choice after the match. Well that's all, I can confirm it works since I myself did it and it popped right when it was supposed to.
Also, as the other solutions state, you have to start and end the match as that character/form. You probably can't just pick Goku, transform to all his forms in a match and take all them out of the list. And maybe if you transform from Goku to Goku SSJ, starting and finishing the match with what the game considers two different characters, it won't count for either. Or just for one of them. So just don't: start and finish with the one you choose.