Depends what you call at certain point of time ...
'would not write the quest at once, but you can start a timer in the ATTACK event, that will call a cast a spell in TIMER event.
i heard the timers were broken in perl quests, but did not figure out. Search the quest creation forum about this.
For more accurate casting, there have been discussions about more event triggers, like when reaching a certain HP ratio, but nothing implemented afaik.
Search quest forum for something called 'Need New Event'
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