Contests about higher multipliers can be fun because your wagering or the amount used for bets doesn't matter, and all that matters is how big a multiplier you can hit. I don't remember very well, but I might have participated in a similar contest in the past, and because I usually wasn't lucky enough, I could barely get a good multiplier, and others were always ahead of me. That's the disadvantage of such contests; if you are unlucky, you will spend so much money on slots but don't get a good-looking multiplier to stand a chance.
If you are chasing high multipliers just for the sake of winning the high multiplier contest, then the best strategy is to make small bets. Just bet with the smallest amount allowed (make sure to read the contest rules, some of them may require a relatively high min bet amount). This way, you will be able to make more bets which means more chances to hit a high multiplier.
For leaderboard contests, which are harder to win, the best strategy is to play games which have the highest RTP or the lowest house edge.