Pk Fire, and a lot of it. Abuse Luma and force Rosalina to approach. Luma has no shield, so roast him alive while Rosalina looks on in horror. Also use Rope Snake a lot, for it is almighty. Kick Rosa in the face while she's trying to recover. Recover with DJ and tether when you can; if you must use PKT, then use it from a distance so she doesn't kill you. Mix-up recovery in general: sometimes recover high, if you can. Rosa's especially vulnerable to vertical KOs, so kill her w/ d-throw>u-air. Don't fish too hard for combos, because she can be difficult to get true combos on or grab. Don't try to absorb default side-b, because it's a disjoint; the custom Falco laser one is absorbable.
In summary, spam PK Fire until you see the victory screen. And be sure to yell out "PK FIRE" irl every time you use the move.