Milotic can be a Very Good special attacker, and a decent defenser.
Having 95 base stat in Hp, 79 in Def, and 125 in Sp. Def, makes it a solid wall, but it is still weak, having only 79 base stat in def, and can be easily KOed by a Thunderpunch.
So, my suggestion is the following for a offenser:
148 Hp, 252 Sp. Atk, 108 Speed. Having 150 HP means it can survive at least 1 hit, 252 sp. atk makes all sp. atks kill the opponent, and 108 speed ma otkes it faster than the opponent.
Heres my other suggestion for a defenser:
252 HP, 252 Def, and 4 Sp. def. 252 hp and def means it wont be koed by a thunderpunch, and milotic already has a very high sp. def, so we wont be bothering a lot with that.
Chose which set you wanna use and prepare to win!!!
Hope I helped!
source: Experience, pokemondb page for milotic