What I did was, check KH wiki's monster and boss info for previous towers. From there, I make an estimate on how much damage I could probably do with my best team, compared with boss HP on each floors, and estimate what my highest achievable floor is. Then using that floor, I calculate how many days it would take me to complete assuming I use 2 keys a day. Set aside himes that meet the requirements, AKA Balance/Defense/Tricky etc. and you will know whether or not they can be used on prior floors. Once you have that done, check which himes you have that provides buffs/debuffs you can use, and try to use them on higher floors.
With the above done, you should easily have a plan for the hardest possible floor you can do, a plan for specific-type floors, and you basically plug in remaining himes into previous floors with weakest himes doing the earliest few floors.
Once you complete the hardest possible floor, throw your other elements at even higher level floors and let them shave off as much HP as you can, then spending an extra key to kill that floor with himes of the right element(fire in this case).
2 things you can do in between events.
1) You can take 2 days off in the middle of the event to allow hime reset.
2) IF you can not fulfill requirement for hime-type of the same element, ignore the "full fire team" and settle with "type" reward. That gives you more tower medals if you only plan on running it once.
2-1) You can run the same floor twice for both type reward and element reward, but the second run(element reward) is best left until the very end where you are no longer able to push higher.
This was my strategy during water tower, which I had less than a week to prepare for. My grids and everything was god awful as I had just returned to the game, rank 52ish and stuff with almost 0 event weapons, not even 100% assault on ANY of my grids, and lack of SR/Rs. My highest record then was floor 10.
So if you have a decent assault grid, and can rotate your himes without much of an issue, you should definitely do better than me(my record during water) with EASE.
Edit: A thing I'm prepping for now, which seems to work wonders for many others, is getting Solomon for floors 1-8.