Magellan Kinjite poison overcomes luffy's immunity. It depends if Luffy can protect himself with Haki or not. Seen as there is not much force behind poison attacks, his haki should hold firm. However Oda, could push it either way.
Take in to account that Magellan was never beaten and would have killed Luffy and Blackbeard in the same day if not for Ivan and Shoryu(? old Impel Down guard)
Though I got to say his "poison" was more like Acid then a poison. Poison has to be ingested to be lethal, venom has to interact with the bloodstream to be lethal, Magellan's "poison" burned to the touch much like an acid.
Well Luffy's immunity to poison is coz he already experienced the worse poisons than Hyouzo's & shinokuni IMO that is Magellan's so since Magellan has more powerful poisons than the ones Luffy was exposed to(hydra/purple) for example the crimson venom demon, Luffy's new found poison immunity will be put to the test for sure but with haki he should now have some immunity to that VD form's acidity, so Luffy extreme diff I think.
That big poison demon of his had the power to bring down the whole of ID.
Plus it spreads on almost anything and just f*cks it up big time.
Luffy already admitted his BH was pretty weak compared to others back in FI so I dont think it could protect him from a poison as lethal as this.