I'm pretty certain this has not been posted before and this is not that big of a exploit but it can give your enemy a little bit of confusion.
What happens is you join the Arena match (1-2 seconds) from when queue finished also make sure you have at least 1200ms latency.
After that you enter the arena match and the gates are already open but at the top of the screen it will say its 1v2 when actually it is 2v2 and since I play rogue warlock in 2s they seem to think its just 2v1 there vs my warlock and rush us not thinking of a proper strategy meaning you may have a better chance of winning.
I have not tried this on 3v3 5v5 but I assume it would work as well.
note. this does not work all the time you just need to get your timing right hard to do i know.