His English is just a thin veneer, so his weaknesses quickly show.
付け焼刃の知識で議論しても、すぐに論破されるよ。
If you argue with superficial knowledge, you'll be refuted right away.
blunt sword with a tempered edge
Original literal meaning: a sword made by welding a hardened steel edge onto a softer iron body, producing a blade that looks sharp but is actually weak. Now mostly used to explain the origin of the figurative sense.
「付け焼刃」とは、もともと焼きの入った刃を付けた粗悪な刀のことだ。
Originally, 付け焼刃 referred to a poor-quality sword made by attaching a tempered edge.