If your opponent has 5 or more cards in their hand, Luffy will force them to trash a card from their hand when he goes for the attack, cutting down on their resources
The crystals are calling will you answer
If the face-up creature had less than 2 power or toughness, then its morph cost could be cheaper
Artifact Creature: Phyrexian MetamorphPhyrexian Metamorph, 155812 Inclusions Enchantment Creature: Nyxbloom AncientNyxbloom Ancient, 82729 Artifact Land: Silverbluff BridgeSilverbluff Bridge, 62657 Kindred Enchantment: Hoofprints of the StagHoofprints of the Stag, 4988 Kindred Instant: Blades of Velis VelBlades of Velis Vel, 4817 Kindred Sorcery: Boggart Birth RiteBoggart Birth Rite, 1813 Instant Sorcery: Flotsam // JetsamFlotsam // Jetsam, 1450 One of the most flexible cards ever printed, Phyrexian MetamorphPhyrexian Metamorph can come down as a copy of any creature or artifact on the entire battlefield, and can discount itself while it's doing so
There are also plenty of straight-up merfolk tribal syenergies that round out the deck's ability to close out games, too