Something that comes to mind is a battle scene described by Spoony in one of his vlogs.
This was an epic fight branching out over a good portion of a capital city and was the second-to-last battle in the entire campaign.
The 'heroes' had, right at the beginning, seriously pissed off a sort-of-a-demigod of a man running the city and they'd been upping the ante the whole time: the man trying to uncover them and flush them out so he could kill them or worse, and the PCs trying to stay hidden and doing a remarkable job of being successful at life in those conditions.
This fight was open war between a rebel movement they'd stirred up in the city, and the mercenary guards the city's leader had called into the city to try to flush out the PCs (who had been doing hit-and-run tactics on the guards up until this point to wittle them down)
Anyway. Fight was amazing, but the most amazing part was the spellcaster in the group.
This is a system in which spellcasting takes investment of a lot of time and resources and cannot be done simply or quickly but when done is very powerful.
What this spellcaster had done was - over the group's week of planning for this final fight - gone around to rooftops everywhere, and started rituals to 99%-cast spells in those spots. They planned it strategically, and planned exits so that the spellcaster could immediately move from one ritual circle to any one nearby and hopefully evade chasers.
So whilst the rebels were fighting in the streets below against the mercenaries, this spellcaster would be running up to one of his ritual circles, spending almost a minute finishing off the spell - then he'd bathe the street in fire and run like it's nobody's business to get away from those who'd take notice and make chase.
Then he'd appear elsewhere and launch off another spell. It was devastating.