After Celty gives you the Seal of the Aurum Blade, she turns into a teleportation crystal. By using the crystal, you are taken to an area Marduk is located in.
The trick to defeating Marduk is to distract him with your blade, then go behind him. For Allen, use the hit-and-run strategy. For Enoshu, she's ranged, so if you stay far enough, your good. For Sigmund, just walk up behime him and start hitting him.
What I did, since I play as Allen, I used my Stone Blade (Don't judge. there's three of them, and they are step 11), and since he was busy killing my blades, he didn't attack me. Then I went behind him and used Cross Wind Cutter and Shooting Weapon.
Also, when he turns into a god (if you didn't already know), stay as far away from him as possible.