GURPS Dungeon Fantasy 19: Incantation Magic

[Phantasm]

Just like with an F-15, the loadout will depend on the mission. In this case, "mission" is "go into a dungeon, kill stuff, take their loot". If, for instance, you know that this is a dungeon loaded with orcs with spears and some magic, you'll probably want spells that deal with taking out a lot of folks at once (rain of fire), buffs, and a few countermeasures to defend against spears and other pointy objects (reverse missiles).

On the other hand, if this is a dragon lair, then you'll want stuff that protects against the dragon's breath attack, if you know what type it is (fire, ice, lightning, acid, etc), and direct-attack spells geared for exploiting known weaknesses (fire drakes might be more vulnerable to ice magic, for example).

Just like you wouldn't load up an F-15 with Durandal anti-runway missiles when escorting bombers, instead going for Sidewinders and AMRAAMs for shooting down enemy anti-bomber interceptors, and maybe a few Mavericks for taking out the occasional SAM installation, you don't want to load up a on explosive fireballs when facing a firedrake that might be heavily resistant to fire; instead, in that case, I'd go with protection from fire, a few strength buffs for the front-line fighters, and maybe stuff like ice dagger.

Clear as mud?