There's the Death spell, which doesn't work on every monster, and some of the stronger monsters take a lot longer to kill through that method. It's notably effective on some of the Flan mobs, as some of the stronger versions of those monsters tend to be immune/resistant to most forms of damage outside of the ring or a specific magic element.
Then there's Holy, which is basically whatever your Defend/Dodge button is. You can hold it, but that pointlessly wastes MP, so ideally you'd just use it right before a monster attacks you, and then Holy damages them as you dodge the attack. So essentially, you can hurt monsters just by dodging while holding the button.
The last skill is called Alterna I believe, used by holding both the Lock-On button and Warp button if I recall correctly, opens up a wormhole or whatever. I think you can only use it when you have your full MP, and gives you a brief window of super armor when using it. I suppose it's an autokill move, but for a lot of the stronger monsters you'll face post-game, they are flat-out immune to it, which is kind of lame, but whatever.
Thanks, makes sense that enemies would be immune to some of those spells.