I hope I am understanding the timer correctly : You get 5 mins to use however you want and then after these 5 mins are over, after using them in various turns during the battle. Now, the player only has 15 secs every turn where he might have to do some calculations, tricky double-switches or predictions which take significant amount of thinking of various scenarios?
I think either the 15 secs need to be increases to 30-40 secs or there needs to be another time-pool where the time which you didn't use out of 15 secs get saved to provide the player more time where he might need it ( something like showdown).