Leveling them all up in order to rank up champs is the best way to get through them in my experience. Going by the fact that you're only two weeks in, you won't have many, if any 60's yet. So I'd just level up your summons so you can rank them up to higher levels and keep repeating that process. This is also the best way to obtain silver, so if you're finding that it's still not quite fast enough, just use the 1 stars for leveling food to speed up the ranking process and in turn use less energy.