If the debris were made of other materials, the Blade Mantis would likely only need to keep spraying its energy exploding particle beam, and it could easily evaporate the wreckage.
Unfortunately, the Spherical Warship material is the Compound-Eyed Observers' own technology. The semi-organic, semi-metallic shell of the ship has a strong suppressive effect on the spread of physical toxins. The beam only causes a minor explosion and does not consistently spread, like it does when it destroys a human fleet.
The Compound-Eyed Observers' own domain has become Harrison Clark's best natural battlefield.
Moreover, within the narrow spaces filled with debris, the "mere" three-meter-tall Harrison Clark can still utilize short-range warp abilities if he controls them accurately. However, the Blade Mantis cannot.
This seemingly unequal confrontation has become a one-sided slaughter for Harrison Clark.