Teayre

There's thorns too.

Okay.. let me put it this way.

Depending on the weapon that Darkness Blade uses, Zero would have a multitude of sword arts to go with.

Now, lets look at Regen and Battle Healing:

BH = Heal (4xX) % after 10+ CD
RGN = Heal X (2 x number of slots) after 6 CD, and only after enemy hits.

Let's say, Opal was Lvl 25 which has 100 HP.

BH at each level would heal 4/8/12/16/20 hp
RGN at each slot would heal 2/4/6 hp

Zero can probably go well above all that damage.  To defeat him, would mean believing in Cardinal-chan.

Thats pretty simple then. All you need to do is take his sword away! XP