I know, it's irritating, right? I almost got 15 tries on that when I was still weak in
level. But here is what you should do:

*If you are level 25 or higher, or if you have some high-leveled monsters, you should be
fine, but if not, here are my tips:*

::By now, you have Zelos in your party, and I know that he has a special skill when he is
now in the Cape Fortress, so equip it and use it in battle, if you do not know how to use
a Special Skill with Zelos, just Press B while doing an Arcane Arte or while performing
High Spells.::

::DO NOT depend on just doing a Free Run so you can dodge Decus' attacks, because Decus
heals pretty fast, and he heals without performing a spell. It's SO irritating, right?
:))::

::Every time Decus does an Ausburgh Arte, dodge it, because that is his most powerful
Arte. That's what's irritating me, because it always makes me die, which is the reason why
I always run out of Life Bottles when battling with Decus.::

Well, those are my tips. If you do these I suppose you will win. But if not, it isn't bad
if you try again.

**SPOILER**
You know, I know battling Decus is hard, but you should get ready for Alice+Decus! So
level up until you can! :))