The high level spells, particularly the instant death types and the very high damage spells, are useful for a number of reasons.
Against lower level creatures they're great since it means combat tends to only last a couple of rounds, if that. Against higher level monsters a lot of the time even one kill from a group of 8-9 can be a good thing.
I tend to leave the level 7 spells except against certain types of monsters - Rapax are fairly vulnerable to Water spells so Tsunami and Blizzard can be very useful. You'll often find that there's at least one realm that monsters are vulnerable to and that's where level 7 spells come in handy.
In general it's the same story as the other magic spells in the game - direct damage spells are often not as good as buffs or status ailment inflicting spells.
xfactor
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