Okay, so first off I will admit that I completely fail at this strategy-type stuff, especially within the Pokémon games, as it isn't really something I've tried to master and make the most of. I mostly just play the games to have fun. However, I do know most of the basics so that counts for something, right? Well, anyway, I'm gonna attempt to explain some things I've learned on how to get the most out of your Pokémon––in this case its Milotic––in all ways possible. Here goes nothing!
First off, you'll need a good nature. Depending on your personal preference you can go for a Bold nature, which decreases the Attack stat but increases the Defense stat, or Modest, which decreases the Attack stat but increases the Special Attack stat. Either is a good option as you don't really need the Attack stat, but if you're concerned about Milotic's low Defense then Bold is the way to go. Another thing to keep in mind is Milotic's characteristic as this will affect its stats as well, albeit in a minor way. Still, it is best to find one with a characteristic that would increase its Defense or Special Attack stats.
Another thing you can make use of is breeding. I haven't entirely tried this out myself yet, but it is good to use for passing down moves that Milotic would otherwise not learn on its own, like Dragon Pulse or Mud Sport for example, plus there is the chance it'll know its Hidden Ability, Cute Charm, instead (though personally I wouldn't go for it). Also if one of the parent Pokémon has a nature you want to pass down be sure to give that Pokémon the Everstone to ensure this will happen. The gender you'd want for Milotic is pretty much up to you as the only difference between them is the size of the ear fins.
Lastly, is the moves you will teach Milotic. This part is pretty much personal preference so you can teach it whatever you want. In Alpha Sapphire the moves I taught mine varied depending on whether I'd do contests or not. Currently my Milotic knows Surf, Dive, Ice Beam, and Hidden Power. I don't really remember what I'd teach it for contests. I think it was Water Sport, Water Pulse, Hidden Power, and Ice Beam, but it has been a while so I don't completely remember.