During the game you will at least few times meet an opponent that you won't be able to defeat, even after more than one try. It might be a dragon or very strong group of ogres. Dragons can be found, for example, in cave in Siring Falls, Temple of Hylea or on lower levels of Od Nua. Below few conditions have been described, meting which should help you prepare for battle and lead it in the right direction. Of course, you can use that tactic on every opponent, but it will consume massive amounts of time and resources.
Just before going to battle, let your team rest - in the fortress or an inn. Choose more expensive options to gain more long term stat boost.
First pick a character with highest stealth level. Try to find enemy position, whether he has some helpers etc. Remaining in the scouting mode, set at least two traps, the stronger the better. It will give you advantage from the beginning of battle.
Feed every member of the team. By doing so, you will increase their endurance and other stats.
With crafting, create potions and give them to all your companions. Develop some offensive scrolls and equip companions with high lore level with them. Don't be afraid to use potions and scrolls during battle.
Your companions very often have skills that buffs other party members. Some of them can be used before battle starts. Priest has the most of such abilities, but other classes have supporting spells as well.
Position your party in the right way. Each member of the team should be as far away from others as possible. Even ranged characters shouldn't stand close to each other. It will make enemy attacks with area of effect less effective.
During battle, position fighting characters in a way so that each of them will face enemy from different side. Whenever one of your characters start to lose health too fast, retreat him to safe distance until enemy becomes interested in someone else. Only after that return that character to battle.
Use all available abilities of each class. Be careful with spells and try to avoid the ones with area of effect. At the beginning of battle use mostly abilities that weakens enemy.
Use active pause as often as you need it.