Gah. My reply was just deleted. Ok, from the top:
1) The range is not omnidirectional. It's substantial, but not omnidirectional.
2) You'll notice that only breathing creatures die from it, because it's like a pollen they inhale... trees are totally unaffected for instance... so, this raises some important points; 1) does someone who turns his body into water during fights need to breathe? Probably not, in which case he's apparently unaffected for some reason (if we go off Tobi's explanation, and the background damage seen). 2) If Mizukage is doing his usual routine, and hiding behind a mountain or something, won't the pollen all be buffeted off it, just like the wind is? Mizukage might be able to dodge entirely, and could be a long way from the pollen.
3) Mizukage will know to dodge far away if he can (though he'll probably be expecting an explosion) because it's similar to his own clone exploding tech, so he's hardly going to be standing around waiting for it to hit him. For all we know he'll be using a basic jutsu to hide underground, or in a nearby cave, or inside his clam or whatever.
4) As soon as the fight begins, his team mates will try to kill him. This will make it hard for Deidara to use C4. They will continue their attempts as he tries to get distance with them, since they will think he's the mizukage.
5) To do this, he has to kill everyone else as well... I don't recall specifying bloodlust mode turned on. You may as well eliminate his team, since he'll be killing them all to do this.
6) Deidara trained his eye to "see through the sharigan... especially it's illusions". That doesn't mean he is immune to all genjutsu, especially not a mirage which doesn't even require eye contact to activate or function. A mirage that is so powerful, the sensors who found the Mizukage after it was dispelled, could not find him while it was activated. And just how confident is Deidara supposed to be that he's found the Mizukage, or that he's a safe distance from the C4, when he's trapped in a battlefield wide genjutsu that can distort his reality? Again, this seems very suicidal of him to try.
Forget Deidara for a minute... how do you imagine everyone else is going to fare? Not well I'm guessing.