First thing first , get high jump and double to be at least at level 2. You get these from leveling up Valor and Master Drive Forms.And also not to use summons or limits as you really need the MP for healing yourselve.Stock up hi-potions in both party members ( I recommend using Donald and Beast ).

Strategy on defeating him.
Maintain lock onto him ALWAYS.Have Donald uses his magic often on Xaldin while you and Beast attack him straightforward.If there is a reaction command "learn" , click it fast as you will need to use it often.He is not
as vulnerable as he looks, as the second you get close to him, you will
immediately be met with several lances straight to the chin if you're not
extremely careful. The only reason you want to get close to him while he is
doing this is so you can coax him into an attack, dodge it and use 'Learn' to
get more 'Jump's.

After using the same strategy over and over again , it's time for him to use the dragon like thingy attack.You cannot attack him at this stage and you can't even avoid it.My suggest is to just accept it as it will give moderate damage onto you.After that, keep attacking him with the same strategy.Note : be sure that you do not use your MP for any other reason except healing yourselve.

Anymore ... you can refer to gamefaqs