Level 45 requires you to collect 5 gnomes but first you have to remove obstacles. Although you’re given quite a lot of moves, you should use them wisely.
One of the best strategies is to focus on the lower part of the field. The sooner you clear the boxes and chains, the sooner you’ll be able to create stronger power-ups. Apart from that, clearing the lower part of the field will let you collect the most covered gnome, also causing multiple auto-matches in the upper part of the field. Keep in mind that using multiple power-ups at the same time charges a Rainbow Blast faster, which can be extremely helpful in the end of the level.