If you are talking about pink keys, then keep them. And keep your pero so you can buy n gacha tickets for the next gambit event. If you have more than 1000 pink keys, the one focus enemy will only drop cases. It does take ~1.500.000 pero to get the 1000 keys, but it is better in the long run.
Currently playing: Aigis, Harem Heroes and Crush Crush
Apart from buying nGacha Tickets during gambit for a ton of pink keys, there isn't much else to do with Pero anyway. I know some people sank a billion pero into voting tickets, just because they had nothing else to do with all their money..
Well, there's ofcourse also the option to buy a ton of nWildcards, trade those in 10:1 for rWildcards, and then trade those 10:1 for srWildcards. I think that makes 21,000 for one srWildcard? xD (10,000 pero for 100 nWildcards + 10,000 for trading those into 10 rWildcards + 1,000 for trading those into 1 srWildcard)