This outcome of this fight depends entirely on whether Magellan's poison would dissolve Doffy's string and where the fight is taking place.

Doffy's mobility is his great asset here because the one way Magellan can use his poison is if he hits Doffy directly. Unlike Luffy, Doffy can simply use his strings at long range to damage Magellan, so his main concern becomes dodging Magellan's Hydras. If they're fighting out in the open, Doffy's mobility is limitless. But, if the fight is taking place in an enclosed space, he'll have a harder time dodging.

So due to his mobility and long-range capabilities, Doffy would win an extended fight. But, if he's caught off guard by Magellan and the warlord takes a hit, the nature of Magellan's abilities would mean that Doffy would succumb to the poison rather quickly.