Dragon Ball Raging Blast 2

It is in fact a random achievement. Unlike the above two posters, it took me five seperate summonings before Porunga finally showed up (though he showed up twice in a row). Another thing to note is that resetting and reloading does not work (I tried about six to eight times to see if it had any effect and it does not).

If you're wondering, like the above posters said, you can get the dragon balls from Galaxy Mode. Play through a character's mode until you reach the boss stage (the big glowing circle, larger than the other circles) and when you beat it, you'll unlock one of the dragon balls. Once you complete seven seperate characters, you'll get a full set and can summon the dragon by selecting "Shenron" within the single player menu on the main menu.

One way to tell that you're summoning Porunga and not Shenron is the summoning video that plays after choosing Shenron in the menu. If you're summoning Shenron, you'll see the body of light followed by Shenron's head. If you're summoning Porunga, you'll only see the body of light twisting around.

Keep trying at it. It may take a few summonings, but he'll show up.