Get the Dragon Ball's and summon Earth's Shenron four times and get the Dragon Ball's again and then you summon Namek's Shenron.
I don't know i never got that far yet but i doubt u can summon red shenron.
first you have to get all the dragonballs in galaxy mode then go back to the thing that has galaxy mode and it should say summon shenron
after goku summon shenron
find all the 7 dragon balls in a saga the there will be a place on your map that says shenron
Get all seven Dragon Balls,summon Shenron,andlook for "Halo".
go to single play in the main menu, then pick galaxy mode, and pick a character, when you get to the boss with the circle looking slighty different you'll get a new character and a dragon ball, plus every third time you summon shenron, you summon porunga and get 3 wishes.
beaT DRAGON HISTORY AND THEN BEAT SOME TOURNEYS THEN IUT WILL GIVE YOU SOME DRAGON BALLS TO SUMMON SHENRON
It's random.what i do is collect all 7 and save go to the shenron option if your are not on namek reset the game system.repeat until you summon porunga.
You have to save after collecting every dragonball and when you come round to summoning it needs to be in kami's lookout
Omega Shenron was the final enemy of the series so it could be assumed he is stronger. But DB is really bad about keeping consistent with character strengths so its hard to say.
gather all the dragon balls and summon shenron and you will have a small list