The Raikage (A) wins. His Taijutsu is on level beyond Might Guy's. His speed, I'm sure it matches Might Guy's even if he does go 8 Gates and compared to defense, Might Guy has no chance of get through his Lightning Armor.
We don't know what level of power up Gai gets when he opens the 8th gate, whereas we know the full extent of A's lightning armour. Hence, the fight is unfair.