When playing it at-level, Xande's Float will one-shot anyone if they get hit by it solo. The leftover energy being usable for floating on to avoid Ancient Quaga is an unforseen consequence, much like how King Behemoth's Comets can be used to take shelter from his Ecliptic Meteor.
... really, sometimes you just have to give them a little leeway with things like that. Fights would be unwinnable if they were entirely logical; Ozma could just keep eating you until you succumb to Doomsday, Garuda could just fly around and pelt you with wind magic, there's no reason Leviathan should play with us like he does instead of just capsizing our boat, etc.
EDIT
The actual name of Zande's float attack is Imperium, just for the record.