Welcome to Raid Arena - If your team is not the fastest so they go first and then have a turn boost - you are likely f@#$ed.
I personally use Skullcrusher, Big Un, Lanakis the Choosen, and Jareg (lead) on my team and I can beat most teams that are not speed lead or are speed lead have under 90k power (my boys have tons and tons of HP). I can wait for the day I can replace Jareg with Arbiter (on my last 20 missions!)
The real killer in Arena is the combo of Defense Down/Weaken + 50% ATK buff and then some retarded AoE burst that kills your entire team in one shot regardless of who you are using. If you have, for example, an Arbiter that has 300 speed, you will win basically every single battle you attack because you can just sh!t hole the other team in 1-2 turns.
They should tweak how defense down debuffs work in Arena and there would be a lot more viable teams available.
EDIT: If you thin Kael is OP then you have a very rude awakening the higher into the arena you climb. Kael is garbage in G4, or at least people have the means to neutralize him + have way more broken champs.
Wait until a legendary puts a 2 buff NO DAMGE buff on his entire team, then buff strips your entire team, then Torim freezes your entire squad when they buff up again. There are some teams that are legit unbeatable unless you are in like the top 5% of the game,