There are many possibilities, and just because hybrid parties aren't as optimal doesn't mean you can't beat the game with them, even on Tactician. All-physical parties definitely out-damage them, but if you want to mix up your team, you can.
Armor is everything: stripping it off the enemy as fast as possible and keeping yours up as much as possible. Don't focus your mages on the same enemy as your melee guys: focus them on stripping the enemies with the lowest magical armor and let your physical guys focus on the enemy mages and such that have low physical armor. Give all your companions a few options to do the opposite type of damage as they usually do. I'm tweaking a hybrid party and I've given my mages a couple of points in Necro and some Necro spells to do physical damage, and my melee guys have some skills that do magic damage (usually in Geo, but there are a few in other schools). That keeps everyone useful.
In Act 2 it'll become easier to mitigate AoE damage to your melee guys: gear with resistance to various elements will start dropping, and you'll come across more ingredients to make resistance potions. Although you will find some in Act 1--check out some recipes
here.
