When I have a bonus with a wagering requirement, I usually start by playing live blackjack. If I win, I use those winnings to finish the rest of the wagering requirement on slots. The key is to check the info on each slot and pick the one with the highest theoretical RTP (return to player) I can find. I also make sure I'm playing in a way that gives the smallest house edge. I reduce my bet to the minimum while still getting the highest RTP, which helps lower the variance. It's really tough to meet wagering requirements, but this has been the most effective strategy for me.