I found this one really hard at first, because I didn't really understand how Lightning Storm worked. To raise the multiplier you have to have some time "in the bank" when the timer reaches zero. If you leave any time jewels on the board when the timer reaches zero they turn into special jewels.
x4 was easy to reach for me, so when you get there just make sure that you match at least one time jewel and you'll be able to reach x5. I actually left a couple of time jewels up until the end of x4 so that they would turn into special jewels and I could get a big combo as soon as x5 began. That caused time jewels to drop in and I was able to reach x5.