I had a really difficult time getting this one unlocked for no good reason, so I decided to make this for those who similarly struggled.
I figured it had to do with getting the bomb to the maximum fire output, but I couldn't figure out the how. For the BASIC materials, you'll pretty much always end up about 2 or 3 spaces away from filling up the Fire element on the bomb all the way.
Turns out, the best/easiest way I found to do it is to make sure you make some Pure Oil on your own with +5 (or maybe even +4) of the Fire element. You can do this by looking at your materials and using the "Awakened Effects" such as "Awakened: Fire +2" to achieve this. (When crafted, you should see that your Pure Oil has 4 or 5 bars of Fire full.) Doing this ensures that when you go to craft the bomb, it will take care of those few missing spaces and fill the Fire bar all the way to "Fire DMG L".
The final recipe I used was 3 Phlogiston, 2 Pure Oil (one of which had 5 Fire bars, the other the regular 3 Fire bars that you can get from purchasing them from stores,) and 1 Red Supplement.
Honestly felt really stupid as I had been running around for a good hour looking for some "special rare ingredient" that was gonna put me over the threshold. Just getting used to do those extra effects you have to use to buff up your ingredients separately before putting them all together. (For a minute, I thought I was gonna have to wait another year to get to the Arland celebration in December to buy a Fire boost thing!)
Well, I hope that you guys had an easier time with this.