Focus on 5 heroes for arena and forget about coliseum. Spliting recources is a bad idea.
You can succeed in arena with every team you choose, but the one I recommend for earlier results is:
(Leaving enchantress out because you wont need her, the key is to kill everyone with landslide and great sage before the enemy can do anything)
My ideal arena team would be:
Landslide, Great Sage, Toxic Shaman
And then 2 heroes you want. Good ones are: chiron and ambrosia
I use abyss demon instead of toxic shaman. I will see after I upgrade toxic shaman. The good thing with this team is that when you kill one or two heroes you get those into your team and then it is not possible you lose.