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Did you try bashing him when he's eating a corpse? I haven't seen this boss but it sounds like the same mechanic that some monsters, like spiders, use.
You need to learn to interrupt. Any monster that has red Sparks around it can be interrupted. So when you see one sparking you can bash him or use a skill that has an interrupt. For magica characters you can use crushing shock from the destruction staff skill line. It is a morph force shock.
You can bash with anything, or nothing, if you had no weapon and no sheild, you can still bash. That is fundamental function.
I will agree even with bashing, many times for me he had already refilled back to max health. As a tank, I just don't do enough damage. And I am use to needing to bash.