I've got out of the Multiply BTC bonus a few times with a 25x rollover requirement. I don't like using the low multiplier strategy for this, I'd rather just bet normally. Trying lots of low multiplier bets just reduces the probability of variance being in your favour. The higher the number of bets the closer to the expected value you get. That's the same reason bots and scripts reduce your chances. I prefer to make fewer larger bets.
I'm a bit confused with your technique, is it avoid low multiplier and make fewer larger bets?
wouldn't that
(there's a chance) make you lost big amount if got hit by bad long losing streak so are you saying betting normally with 2x multiplier would be better?
why betting higher amount with low multiplier wouldn't work the same?
(towards 25x rollover)is it because the return too small to cover the losses?
The multiplier wasn't the point I was trying to make, I'm just saying I wouldn't change that.
I bolded the important bit of your quote. Yes, there is a chance that you get hit with a bad luck streak but there is also a chance that you get a good luck streak. That's variance of luck. Smaller sample sizes will have more variance and the longer you continue playing the less variance there is. Over a large enough sample size, there will be almost no variance so the result will tend towards the expected value, ie. the house edge.