There's a couple of important things about Deadname which should make the fight easier:
- During his first phase, he won't actually attack you at all - he just counterattacks. That means that you can take the fight entirely at your own pace - if you're low on health, just stop hitting him and heal up.
- Once you drop him to half health, he'll use an attack which will always wipe your whole party except for Sam. Make sure Sam has Life or Half-Life so she can revive the others!
- Once you've got everyone back up, the second phase of the fight is much like the first. The difference now is that he'll start attacking even if you don't hit him first - but he still won't do it very often, so you should be able to get away with using similar tactics. (if you have the DLC, you can just skip this phase with Zantetsuken)
Also, Sam's Slow spell is very good in this fight (as it is in general in fights against a single enemy) - if you combine it with Illegal Fireworks to instantly stagger the boss, you can essentially cheese the fight.