It's hard to give you a quality advice right now because they have changed so many things from EA.
For example what Kalrakh suggested is something which Larian wants to prevent or nerf a little bit.
They suggest to go with a mixed group because they made it so that in pretty much every encounter there are enemies with high physical armor but very low magical armor and vice versa.
However I have my doubts if this is enough to make mixed groups stronger than pure physical/magical groups.
I think the most important thing for a strong group are synergies, especially if you play mages, as there strength mainly comes from their combos.
However there are a few combos for physical char's as well one of the best combo in game was for example DecayingTouch with FirstAid.
