The Behemoths in Final Fantasy IV are the worst. You get the dubious pleasure of having to fight 3 of them just to stand before Bahamut.

You ever try casting something the Behemoth doesn't like? He responds with Maelstrom. If you know FF4, you know that Maelstrom is the worst news your party could see...all of everyone's HP are reduced to <10.

Try fighting with <10 HP. Yah.

The trick is to cast Blink a lot, and nail it to the wall with some physical attacks.

However, as Mr. Yahtzee Croshaw noted, SE does a pretty good job of changing up the combat engines in each new game, so the Behemoth in the Duscae Demo may or may not respond to physical attacks and magic attacks in the same way. Good luck.