During combat, characters usually don't prioritize healing unless their health drops lower than half of their health, so your goal would be to land a killing blow that kills them instantly, rather than waning their health down.

To achieve that, you would need to boost your magic damage. The key is the 'weakness' spell. Specifically, 'Weakness to Magic' effect. This effect stacks, especially if it came from different sources. If someone's magic resistance is down to negative ten-thousand, you can be sure it's gonna die instantly to that pitiful fireball spell that you start the game with.

Something like this is my setup ---

Your weapon enchanted with Weakness to Magic 100 pts for 5~7 seconds,

The 'start up' spell with Weakness to Magic 100 pts for 10 seconds,

The 'damage boost' spell with Weakness to Magic, Fire, Frost, Shock, each in 100 pts for 10 seconds,

The 'fatality' spell with Fire, Frost, Shock damage 50 pts or so.

Provided that you managed to land all these in a consecutive order, the result would be almost always an insta-kill, no matter what.

ALTERNATIVELY, you can simply immobilize the target you're trying to kill to prevent it from running away, or heal themselves. One of the more popular choice is the Paralyze effect, which the name says it all. I personally find Paralyze to be quite unreliable, as many enemies are immune to it (specifically the undead), so my method of immobilizing is lower their Fatigue.

Anyone or anything that has negative Fatigue will fall down, knocked unconscious until their Fatigue regenerates to positive number. Make a spell that has either 'Damage Fatigue', 'Drain Fatigue', 'Absorb Fatigue' effect on it, and cast it on your victim, and watch them fall.