deciding a party...

[yuno]

However, when I tried it before he cast mini on everyone which didn't help, and he can kill everyone in one go.

hence,the white magic "mini".you have it too,so go and use it.when ozma casts mini,use mini on your party members,and they'll be back to normal.in short,mini cancels mini.

I'm guessing Ark Summon would be a good plan too?

with auto-regen equipped,yes,summoning ark is pretty much a good idea.