saito doesn't reduce defense, he ignores it. ignore defense is calculated based on the target's current defense so in the presented scenario, gnut would permanently reduce the bosses defense by 30% and saito would then ignore 30% of the remaining 70%.

in the case of multiple champions that permanently reduce defense my assumption would be that it works similarly to having multiple champions with the cruelty blessing where the maximum amount that defense can be decreased is static rather than cumulative. so even if you had both harima and gnut in the same team, you still couldn't reduce a champion's overall defense more than 30%. that is just a guess on my part though; i could be completely wrong.