I don't understand how to play the game, no matter how much magic power i have when i start the battle with the guardian knights i always lose.
Except for the last guardian, I can help you generally with the others , first check out places where there are lots of civilians and few knights possible as those people will come hunting you after you suck the soul out of 3 people and going to engage you costing you 2 powers with each engagement.
Now on the spot you can just check out and memorizes the people that you suck whether or not they give you either 2 points, 1 points, or less then one points as you leave the area the area will reset, try to suck at 2 people and see who gives out the 2 points and who gives out 1 point and who gives less then one point,
now after you memorizes who to suck to give you the most power in that area do so, and if you're feeling bold you can try to suck the third civilian only if you're sure you can dodge/ escape the knights because if you don't then it will all not worth it.
Usually if you're sucked and gather enough per area that the best that you can usually you triumph and if you have triumped/win over one area you get one point permanent boost in your magic power pool so to give you a + 1 just rinse and repeat.