Do you have the himes and grid to run floor 13 with another element though? If you do, you could aim for Completion+5 or more alive bonus which is 550 medals. Then that leaves Light open for 14. If you don't, then use Light now and do a little math.
2 keys for 14th floor, use one to shave off HP and another to kill, getting Completion+2Offense missions for 550 medals. Only if possible to do it with 2 keys. (275 medal per key) If it takes a 3rd key to complete and achieve both, that's 183.3 medal per key.
1 key to complete a mission from floor 6, since you mentioned survival, that's 200 medal, with a possible maximum 50 turn bonus. 250 medal for 1 key.
1 key to complete a mission from floor 11, not sure which so I'll assume lowest medal, that's also 200, with maximum 50 turn bonus. 250~300 for 1 key.

You use 1 on 13, that leaves you with 3. So how to maximize profit with the 3 remaining keys is dependent on how you assess your teams.

Also, a strategy you can consider when planning for next tower. There are 2 ways to go at it:
Play on days 1-5, rest on 6-7, all himes available on 8th.
Play only on days 1/2 or both, 5, 8. This gives you maximum flexibility as your Himes will all be off cooldown on day 5 and 8.

Another thing is, try to research ahead of time which himes/comp you will be using for each floor. You could use Cobble's guide, or look up previous Tower info on the wiki to know requirements and a rough understanding of fight mechanics. Then make an excel with Day:Floor:Requirement(mission):Himes(by R/SR/SSR).