Are you using the best tactics? Whenever I see someone complain how fights are impossible, I wonder how they explain the success that other people have.
Personally, I've beaten the game with three different sets of builds, all on Tactician. There are a variety of strategies you can use, but there are a few common threads to all of them.
The single most important quality is battlefield mobility. Each of your characters should have at least two leap spells -- i.e. tactical retreat, cloak and dagger, phoenix dive, etc. You can usually get the necessary skill points from gear.
Necromaster is not a good primary skill set in my opinion but it's great to have a point or two there for any intelligence-based character. Mosquito Swarm is so good, and it heals you when you are decaying. It's also good to get some healing when you do damage to vitality, in combination with Liquid Armor talent.
Your characters need to have balanced physical and magic armor. For strength- and intelligence-based characters, that means you will want to use some wizard gear alongside your fighter gear. Well, especially wizards using fighter gear.
