You should definitely use a character which you are comfortable with and which you can use to full effect- the hardest part of the Rainbow Guardian is the waves of smaller Gnomes which assault you while you're trying to deal with the Guardian so picking something which you can survive with is key and that's going to vary from person to person depending on what characters each person has available.
Some key suggestions are to use the Bot/Pot spawning points to your advantage but save a couple so that you are able to put down Dr Heals or Heal Flowers, also try to keep the Guardian as far from you as possible so that you can use the columns in the room to block his incoming fire. Don't be afraid to simply stop and flee across the room so that the Guardian and the Gnomes all wind up behind you, approaching from pretty much the same direction so that you don't have to worry quite so much about attacks from the flanks and rear.
Try to take out the smaller Gnomes while keeping columns between you and the Guardian because one of the biggest threats is being swarmed- once you've cleared the immediate threat of the little guys then focus on the Guardian until the next wave of smaller Gnomes spawn.
