The only reason he easily vaporized the slime was that he dispersed its mana body using his own mana breath. But that was because it was too small and weak.
There was no way the same method was going to work against someone like the Hell Maiden. If it was indeed a 'she', she would be too powerful for him to vaporize her. Or maybe that was the solution indeed. But for that to work, his flames should overwhelm hers. Otherwise, he would simply make her stronger since she had the same elemental power as him. That was another annoying strength of a slime. It could gain strength by feeding on weaker flames.
But the dominance of his flames depended on his bloodline.
Now he realized why it was said that the 7th summoning would truly determine the bloodline quality. He felt there was no choice but to gamble all in, though he was glad he took 1 month to prepare for this.
But there was no more time to think as the dark rune circle finished summoning the Hell Maiden.