The Thermonuclear Smoke Devil is a relatively simple boss with very few tactics involved. Thermy a max hit of only 8 but has a very high attack speed.

You can utilize the ‘step under’ technique in order to take less hits from Thermy. What you should do is hit Thermy once, then immediately run under him so that he is unable to attack you until one of you move. This will give you a couple of seconds to eat if you need to, and then jump out from under him and immediately return to attacking. In this manner, you will drastically reduce the amount of damage you take from Thermy and extend the length of your trips considerably.