Well, more of a mini-boss – but it doesn’t really matter. I highly would prefer you stand for this (it is much easier). Raise your shield and target Scervo. Shield bash him as he tries to hit you, then hit him a few times with your sword. Continue doing this to push him off of the plank.
As he gets close, he’ll also pull out an electrified club. At the end of the plank, he’ll push you back and the gate (which damages you if touched) will move closer to you. Repeat this same process a few times to end the battle, making a chest appear.