In order to accumulate a 5x multiplier, you have to pick up 25 prize bubbles in quick succession. The later levels in the game are actually much easier to achieve this on - I unlocked it randomly on one of the Progress Emporium levels.
If this trophy is causing you trouble, however, on the planet Space Bass there is a side-level branching off to the right of the final level (The Funkhole) called 'Full Tilt'.
Full Tilt is a pinball table which contains hundreds of prize bubbles - easily enough to rack up a 50x combo, never mind a 5x combo. :)