Use whatever spells you have quick access to. Reward XP on the magnitude of challenge vs. the stats. i.e. if the fight is challenging, reward an appropriate amount of XP, if it's a cake-walk, less.
NPC/Monsters are designed to survive about 5 or so rounds of combat. Any more than that is, honestly and personally speaking, boring. If it's an epic end battle w/ boss, there should be tons of peon and minion monster... tons, to extend the combat over a longer amount of time. Nickle and dime-ing a monster to death is tedious/boring. With that, make sure if they're a spellcaster that they have big spells (appropriate to their level) A fireball will be a SUPERNOVA, Magic Missile will be Magic Carpet Bomb, etc. at higher levels.
I used to let special NPCs use DCC RPG versions of spells. But I've simplified it a bit with some house rules and use simplified versions of spells that are scaled by the HD of the spell casting NPC/monster (shameless plug, Crawl! #3.) I don't bother letting NPCs have as diverse a range of spell effects as you would regular PCs, there's no point. I pick one basic "misfire" and one basic "crit", and have the spell effect be the typical effect. Much simpler to write our spell caster "powers" that way. I also use the same name to emphasis they're compatibility with counter-spells and spell duels.
A lot of the times I just wing it, and I favor or lean towards "cool" and "fun" when in doubt. If it makes your players cheer, clap, high-five or laugh you're probably doing it right.
If they dance the boogie-woogie, is that alright too?
Thanks for the thoughts, that's great advice. I don't use XP, but apart from that, all of the above is applicable. Being able to mine older versions of D&D for monsters and spells for NPCs will really simplify my life a ton.
If I play at least a bit of DCC, I'll seriously consider following your shameless plug to its root and jumping onboard Crawl.