Lol, that was impressively bad luck.

Blaze is certain that if they can force contact between the bane concoction and the plague spirit it will suffer grievous wounds, which can be enhanced by using a weapon. He also knows that if they fight the spirit in The Hisil it will be more powerful, but will be basically unable to escape them, where as it can more easily escape to the Hisil if they fight it In the Flesh.

Lastly, Blaze has heard rumors of how hard it can be to kill a spirit, and sometimes a defeated spirit is able to come back after a long dormancy if it is not "killed to death".