Devilkiller |
A cat familiar who delivers touch spells for a 3rd level caster at the end of the dungeon might be kind of scary and could potentially have enough hit points to get away and come back later to apply another spell (preferably while the PCs sleep or in the middle of some other combat)
Alternately the familiar could be of the Figment archetype so that no matter how many times the PCs kill it the darned thing just comes back again the next day.