To clear the small area in the center, you need to break the crates that block the entrances to the portals. Once the portals are cleared, you should use the firecrackers to get to the dynamite in the center.
Do not spend too much time on any particular crate while trying to get to the portals. If you notice that you don’t have much luck destroying it, simply move on to a different part of the field. You need to balance out your moves and use different tactics. Power-ups, Lightnings and automatic matches might all come in handy on this level. Be flexible and adjust your strategy as soon as it becomes clear that the current one isn’t working.