Like the Beta Behemoth, the Alpha Behemoth looks scarier than it is. The first time you fight one, your party will have pretty limited options, so just keep pounding away and heal when necessary. Physical attacks work bettern than Magic attacks, but either will work in the long run. They're actually a bit tougher in Chapter 5, since Hope has no great attacks to use against them, but by then you should be able to easily survive their attacks.