Like Vgr says, the status Regen doesn't really have an element (even the spell of the same name that gives the Regen status doesn't have an element); I have a boss that sets the player party to nullify the restore element but Regen can be applied normally and will perform it's effect. It won't kill Gi Nattak either, it'll restore his HP (I tested this once and kept track of it through Sense).