I think third raikage is one of the worst matchups for guy. Guy is all about straight on physical attacks and raikage specializes in tanking these attacks and is known for his extreme stamina. I don't think guy can stay in the 8th gated state for more than an hour, and he is fighting a guy who fought an army for three days. 7th gate afternoon tiger shouldn't be more effective than the rasenshuriken naruto threw and this is because:
- Kisame's body would have been disintegrated by a rasenshuriken, or he would have at least died.
- Afternoon tiger didn't even knock kisame out, it beat the hell out of him, but he was still able to talk after taking that attack head on.
- Kisame didn't even try to dodge the tiger since he thought his shark could swallow it.
- rasenshuriken, even being the third raikage's elemental weakness, still failed to mortally wound him, and also would not have hit him if naruto didn't redirect it twice and work his way into point blank.
That being said, I can't for sure say how the raikage will compare to 8th gated guy since he has never been shown, but if a seventh gated guy can barely do anything, I doubt an 8th gated guy will be able to kill him.