So, I've been using this set on the beta PO gen 5 sim
Milotic@leftovers
Marvel Scale
Calm
252 HP, 228 Def, 16 Sp. Def
Rest
Sleep Talk
Boiling Water
Dragon Tail
And so far I have to say I love the set. If you can get set up and in control you are golden for the match. She can even take an energy ball from...it was either a Shandelaar or a Zoroark. Dragon Tail is so trolly and with marvel scale up she takes so little damage. And the burn rate on Boiling Water is just perfect. I've had it when I didn't need it and had it when I did need it so far.
She will get absolutely crushed from any decent physical attacker without Marvel Scale activated though.
EDIT: After a few more battles she is making people straight up forfeit after she scores key burns.