For the most I've found you just need the right debuff and you can immobilize even the boss mobs. Add in the right summon and you can keep the bosses locked down or busy. I haven't done this right in a while, but I think you can freeze (rain + cold wind) and stun (blitz bolt) the Ghoul. A fire elemental can't be hurt by the Twins and vice versa, but they will waste their time engaging each other if nothing else is convenient to attack. This sound funny but I think I froze the Twins as well at one point. Also as mentioned above ice bolt will mess him up (lots of damage).
For Braccus Rex I came packing multiple teleport spells (a scroll helps, too). Pelt him with arrows and magic, and when he get close, send him far away. Rinse and repeat.
Of course, as others have suggested the best recipe is more levels. Even having one level above your opponents makes a huge difference in your ability to fight them, same goes for one level below.