You can't negate it I don't think, but shell halves the likeliness that it will work. This is especially useful against Yaizmat since otherwise his deathstrike has a 100% success rate.

I had trouble with the necrohol until I set up my gambits so one character buffs all with Protectga, one character buffs all with shellga, one character buffs all with hastega and all characters buff with bubble. It makes things a hell of a lot easier.