[Kameron8]

The issue I have with maximum HP percentages is boss enemies. That 25% HP could be quite a lot for them. That said, they could be made immune.

Don't really like the idea of a percentage based execute

Could you expand on this a little more? It seems far less situational to adjust the static value to a percent, than to deal with the wide swing of flat numbers (See: Ether Invitation). Requiring a boatload of curses to achieve decent invocation numbers feels clunky in my mind, but I'm not sure if this spell is necessarily sticking with the identity of a map-wide execute. With that in mind, an idea that builds on a previous one in this thread -- without introducing removed from battle mechanics -- would be the following:

Enma's Summons

Curses all enemy targets for (Rank) Rounds, reducing their HP to 0 if their current HP value drops below (Rank*4%) of its Maximum value. Additionally, enemies who fall to 0 HP in this manner have a chance to be inflicted with Interference for 3 Rounds. (Status infliction check) Bosses are immune to this effect.