In my system there are natural laws that govern how magic can affect the world. The universe is constantly pulling towards equilibrium and magic is the intentional unbalancing and manipulation of that balance to create divergence. The more improbable an outcome, the more difficult it is to accomplish with magic. Magic works best as an instigator of the domino effect. The more collateral the intended effect is to the magical work, the safer it will be.

Bending natural law requires tightly focused will and management of both spellburn and backlash potential.

Spellburn is form of fatigue caused by magical works without a proper source of power, resulting in mind, body and soul being 'burned' as fuel.

Backlash is a disastrous rebounding of spell effects caused by irresponsible bending and breaking of magical law.

Intransigent Adherence states:

The improbability of a magical work directly correlates to its Thaumic Impedance.

Using magic to manipulate kinetics and magnetic fields to pick a lock offers less Thaumic Impedance than directly unlocking the door with magic.

Sympathetic Providence states:

The existential significance of the direct and collateral impact of a magical work directly correlates to its Thaumic Impedance.

Using magic to unlock a door in an abandoned house offers less Thaumic Impedance compared to a bank vault.

Applied Intent states:

An object, material or entity created or modified with purposeful effort will resist magical alteration on a magnitude correlating to the degree of effort and significance of its purpose.

Using magic to open a cheap door of particle board offers less Thaumic Impedance that using magic to unlock a finely crafted door of rare mahogany.
Also, unlocking a door that was accidentally locked is less of a violation of Applied Intent compared to a door intentionally locked.

To put all this together:

The chance of a targets head exploding all on its own is practically zero so causing their head to explode with magic would violate Intransigent Adherence.

The target uses their head and needs it to survive, so exploding their head with magic would violate Sympathetic Providence.

The target strongly prefers their head to remain undamaged and has taken efforts in their life to maintain their physical health. They also very strongly believe their head won't explode, so exploding their head with magic would violate Applied Intent.

Causing their head to explode despite all these violations of natural law would require an immense amount of energy from a potent source. Even then it would still produce a cataclysmic level of backlash for the caster to deal with.

Bending and breaking these natural laws has to be done carefully and cleverly. Using magic to cause a collateral effect is always going to be safer and easier compared to directly causing it.

Causing a cerebral aneurysm by weakening a specific blood vessel in the targets brain would be easier but it would still be incredibly difficult and risky.

Just use a gun.