Here's a tip when playing Krillin:
RUN,try to be sneaky,he is pretty fast at the start which can be your advantage.Do everything sneaky to gain PL.
Trunks:
IT'S ALL ABOUT THE BURNING ATTACK,melee,burning attack,repeat
Buu:
GO FOR THE COOKIES,you'll most likely lose a lot of health,so get one if you have to,if anything else go for Donuts.They increase PL by 10,000
Vegeta:
MELEE EARLY.You hurt the most at early game,melee the best you can,zanzoken,block,or even kiblobs.Be ruthless when playing this character.
ALL:
DON'T WHINE,just play,don't complain.It will help you.If all else fails,try bots.You can get them from the link in my sig if you do not have it or have an older version.