[Snake]

Thing is Derg, read up what Ether Invitation does. It uses missing HP to damage on a 1-square range 'basic attack'. You can't special sauce the gun's gimmicky thing of ignoring defense for a skill only that forces it to behave like a 'basic attack' (I think?).
The difference between this to the others is that EI is not an autohit, it won't do pitiful damage to people like Cripple Leg since it doesn't use the basic damaging formula and uses missing HP instead, but will work the same way around, they can still be Overcharged for +50 Hit too. The best solution for now is to remove guns from EI list since they can be so easily exploited in-game and horse-kicked thematically. (So bashing people with a gun's grip hurts more than being cleaved in half by an axe if you're wearing armor...?)