I've been playing for about a month and still haven't came across any legendaries but what would be the best setup without them?
I have an all-elite team for both the arena and raids, I'm running Exec, Assassin, Werewolf, Cyclops, and Pain-Da. They're not really the ideal team as I don't employ Ranged heroes (marksman) or healers (angel), but I've had pretty good decent success vs. non-legendary teams or even against players with 1 or 2 legendaries on their team.
Though if you want a well-rounded arena team, I'm on the same page as PhinalHour's suggestion:
Executioner (main attacker, better with berserk talent) - poor man's Champion
Assassin (fast attacker) - poor man's Ninja
Marksman or Engineer (ranged) - poor man's Succubus
Angel (healer) - poor man's Druid
Werewolf (defensive tank, slow attacker) - poor man's Paladin