13th Age uses the middle of three stats for both AC, Physical Defense (~Fortitude/Reflex save), and Mental Defense (~Will save).To avoid the obvious PC-side power creep this idea otherwise produces, I'd suggest using the lower of those stat pairs for the save.
They also have most strong debuffs/crowd control spells based on the target's hit points, which means an enemy will be more susceptible to e.g. cause fear once you've knocked them around a bit.