If you're party members are all in the late 20's for this battle you'll be fine. Here's a good strategy for beating them. First of all you NEED Sveta's Beast forms so put her in your party. Now you're in battle. Use the Venus Djinn Gears to set Chalis on Blados. You wanna focus on getting rid of him first as he's much stronger. Then the next turn use Chasm (Venus) to lessen their damage on you. Keep battering them with effect inducing Djinn so that they wont attack you as much. Build up to some powerful summons. I beat them by using Thor,Judgement,Meteor and Boreas. Just use that method of Djinn and summons and you should be fine. Here is what you should know about them.

Blados: HP:1,931 Weakness:Fire

Chalis: HP:1,830 Weakness:Water

This is my first boss guide type thing so if it gets submitted ill make make more.