It used to be a free power up.. Now as usual you have to pay for bundle packs to get it. The developers of this game are the most abusive to their target base I've ever seen. This and many other reasons are why I stopped playing it
The Slow motion is the timer powerup on the top right when you first start the level. The cost of it varies depending on what difficulty your playing the level on. But luckily the Timer powerup (This is what I call it) Only costs 6000 coins for all 3 difficulties.
You have to pay for the whole power up package & weapons to pass... Impossible without pay & spending the whole power ups, and still very difficult, money hungry bastatds... SCAM!!! unistall