Core rule
Spell attacks and saving-throw spells
Some spells require the caster to hit; others require the target to resist.
A spell attack compares the caster’s attack-roll total with the target’s Armour Class. A saving-throw spell compares the target’s save with the caster’s spell DC.
An enemy can have high Armour Class but a weak saving throw. If attacks keep missing, consider a spell that targets that weaker save.