If you're into call of duty zombie modding for a bit, maybe he can tell how he did this in bo3, it's probably similar to bo1, due to being same engine and mostly same console commands and such. This is a mod for zombies on bo3 that replacing all characters with kermit and has custom voice lines when the mod is loaded.

but when I did a search, I found a mod on black ops 3 which does similar to what you want it's not applying some percentage for reducing zombie health but it caps the health to a certain point like round 10 or 20.

actually black ops 1 is not a good game for custom zombies content, due to its buggy mod tools. The listed mods are only for use outside the leaderboards. DLC included) Menu with the most options ever for Black Ops 1. That includes custom characters, weapon & perk packs, zombies counter, visual effects, mod menu etc. Dynamic Build System (Play on each map with their weapons, entities, effects. I think black ops 1 doesn't have a mod which does what you want. How to install and play Bo1 Custom Zombies: Guide NOTICE The use of any extra mods apart from the custom map itself is NOT ALLOWED.

Call of duty black ops 1 zombies mods