There are different strategies for arena. The easiest way to start is, building a speed team on your own. A speed team has 4 components: speed buffer, attack buffer, enemies defense debuffer, attacker.
1. Speed buffer, a champ that ensures yor team goes first with speed or turnmeter buff. You will get a great epic champ for that as day 30 log-in reward, High Khatun. For now you could use the rare champ Diabolist, that can be farmed in campaign area 12. You don't have to lvl up Diabolist higher than lvl 40, you won't use her later on. She is just a filler until you get High Khatun. Give your fastest speed gear to this champion. Other stats don't matter, only speed, speed, speed.
2. Attack buffer, a champ that can increase the attack of your team. The rare champ Medium can do that, you can farm that champ in campaign stage 4. That champ needs enough speed to go as no.2 of all champs, other stats don't matter. Lvl 40 is enough for that champ, you will champs that do the same thing better than medium later on and shouldn't invest to much in her.
3. Decrese enemy def champ, a champ that can place the 60% def reduce debuff on all enemies. Aleksandr can do that job. That champ needs enough speed to go as no.3 of all champs and accuracy to place his debuff. Other stats, like damage, are secondary.
4. Attacker, a champ that smacks hard and hits all enemies at once. Ithos can do that. His A3 allways hits critical, so you can ignore critical rate at the equipment of that champ and put the focus on critical damage and attack. Still needs some speed to go as no.4 of all champs, without enemies interrupting. Use gloves with critical damage as mainstat, armor with atk% as mainstat and boots with speed as mainstat.
The fight should take only one turn, and all enemies should be dead after this turn without ever making a move.