like i pretty much must be doing something wrong because every encounter atm is like, do as much burst damage as i can before all my party members except eder die and hope that he can tank the enemies for long enough to kill them
My tips:
Keep Eder and any other heavily armoured people at the front of the party, near the beginning of the battle. Have them engage the enemies and soak the damage, so your weaker party members can freely act or reposition themselves. Use per-encounter spells as often as possible, and per-rest spells whenever necessary. Focus on keeping the more heavily armoured party members alive until the melee enemies are dealt with (unless there's a more pressing threat). Avoid using lightly armoured party members in melee combat unless they're flanking (i.e. not the target of) the enemy in question.
One important thing to keep in mind as far as Eder or any other tanks are concerned is that they need high deflection (physical attack defense) and high DR (to reduce the damage they inevitably receive). Because armours' specific DR varies a bit, pay attention to who is resistant to what, and equip them accordingly.
As far as defeating enemies, pay attention to an enemy's tooltip stats as you attack them. Defenses will be revealed as you test them, which will reveal the quickest way to defeat them. Monsters will have their stats fully known once the lore entry is complete, but human/etc enemies' stats will only be revealed as you attack them
every time. Paying close attention to a human/etc enemy's armour will give you an idea of their resistances, as they're wearing the armour they drop; this means mages will almost always have worse defense than full-fledged warriors.
One last thing: keep in mind who your most lethal party members are, and try to engineer situations where they can take full advantage of their skills. If, say, you have a ranger with a particularly lethal bow, try to weaken enemies' deflection or DR so they can get more full hits in. But, again, always pay attention to an enemy's DR; a fleet of full-plate warriors will take little damage from piercing weapons, so try to have a variety of weapon sets for your party to maximize your effectiveness on a per-battle bassis.
Hope this helps.