You can only battle ID when you get the Weapons Room key from Citrine. ID is located in the CAT Testing Grounds on the E.V.S system. ID is a bit hard to defeat. Here is the best strategy I could come up with.

Before You Battle ID - I would wait to defeat ID until you get passed Abel's Ark (which has a boss battle I will be covering soon) so you can acquire Heaven's Door while you are at the CAT Testing Grounds. I also recommend buying about 80 Half Repairs, 50 All Repairs, and 99 Nano Repair Ms. Make sure all of your best E.S. equipment is equipped. You will also obtain Anima 3 when you defeat Abel's Ark, which will come in handy while battling ID.

Step One: When you first start battling ID, he is about as strong as an eleven year old. ID may seem like an easy boss at first, but once you are done defeating his first form, he will transform. You should not have any trouble defeating ID's first form. If you do, than there is no way you can beat his second form, even with help like this.

Step 2: ID's second form is the hard part. The following strategy was the one that worked best for me.

Unless the E.S. Asher is down and out, have him in the battle. The other E.S.s you have in the battle don't matter. The only thing you need to look for to decide which other E.S.s you should use is how much MAX Health it has. Sometimes there will be a blue arrow pointing to an E.S. which you can see once you use the E.S.s. If it is pointing at you, odds are, ID is going to beat the crap out of that E.S. Use the Guard command as the E.S. that the blue arrow is pointing to as that E.S.s turn.

Let the E.S. Asher be the only one to attack ID. Only use far distance attacks! Let the other E.S.s heal all the other E.S.s to keep you in the battle. Once the E.S. Asher has a full Anima guage, use his special attack. I would save the Anima you get until you have three full Anima guages.

ID will use an attack called Demon Lord if he starts to charge. You will get a heads up message when ID is charging. He will attack on his last turn. When he charges, I don't care if you have three full Anima guages, I don't care if the blue arrow is not pointing to you, YOU MUST BLOCK! Repeat this process until you defeat ID.