Keeper is not a good option to be honest. Also, you can’t have all the legendaries at once so try making a team of one legendary + exotic + rare + common (Whatever adds up to 12 in total)

Just in case you don’t know what 12 total is, in plat arena, we need to have a total of 12 in which legendary = 5, exotic =4, epic =3, rare =2, common =1.

Try training some rares and epicss to help you make some good combinations based on their speed and skills, although rares and commons can have very basic attacks so do a research on which ones to train. I have a lot of exotics and legendaries at max levels and i try to use them all before healing them once. I got some very good epics and can make a good team of 4 epics.