Toxic is the best option but unless you transfer Milotic from a previous generation, this won't be a thing so here are some alternatives
Haze. This is probably one of Milotic's best moves as resetting stat boosts never hurts. The problem with haze is that it is an egg move so unless you are willing to breed it, this probably isn't the best
Dragon tail. This is a good alternative to haze as it still gets rid of boosts but at the same time dealing minor chip damage. Of course this does come at the cost of not hitting fairy types
Coil. This is also another fantastic move. A great benefit of coil is that it patches up Milotic's pretty meh defense stat and makes it surprisingly bulky on the physical side when combined with marvel scale burn. If coil is chosen then you can also pretty much drop ice beam and replace it with blizzard since after an accuracy boost, it's over ninety percent accurate or something close. It also allows you to use hydro pump instead of scald and even though it doesn't burn, it's gonna hurt
