I don't exactly want to spoil you everything (that would ruin your pleasure).
The world is very open, so you are free to roam and battle whose level is higher than your. But if you see yourself fighting monsters whose level is above yours and if you do not enjoy it, it's likely you skipped other areas: if you do everything in the "right order", you are always at the same level (or even ahead) than the enemies. Simply get back and explore elsewhere.
Important trick for tactician mode would be to CC enemies (knock down, charm, freeze, slow, ...). You can also use summons as meat shields.
When you explore, separate one character from the group and use him to explore, that way if you encounter a deadly trap, at least only one character is dead and not all the group.
Leadership (for initiative and resistances) and Crafting/Blacksmithing (for money, weapon/armor crafting, consumable crafting, and boosts) are quite important to have on at least one character. Leadership is best on your mage if you have one, you can dualwield two leadership wands and have a leadership helmet to already have a +3 boost. 4, 5 and 6 Leadership are three good values.
Crafting/Blacksmithing, you only need 3 on one character outside gear (belt and bracers give together potentially +2 to both skills).
