Attacking the inside of a monster mostly covered in armor is a pretty logical way to go about it.
The result wasn't normal though.
Expecting the lizard's metal skin to cause it to get cooked from the inside, but nope, it exploded.
Judging by the comments from the witness, the "normal" method is by cooking its insides.
As in "heat the outside, then cook the inside" - meaning probably it can diffuse the inside heat through the skin if the outside is cooler, so you have to prevent that.
Or you do like MC and just overload its capacity by going nova, causing instant vaporization of it..