I am currently going through the game a second time and I had a hell of a time with the Boost Guardian the second time round as compared to the first. All I can say is when he's in solid form, make sure to side boost around him, jump while he turns to a temp. puddle, and nail him with Charged Light Beam. When he turns in the boost ball, turn into a ball and hide behind one of the four pillars and just keep dropping bombs for him to run into. When he turns puddle form for longer, stay in ball form and keep bombing him but stay far away cause thats where he'll do the most damage. Its probably the most annoying boss battle of the game and I know I had to do it about 9 times this time through. Just stick with it and good luck man.