How to apply temporary stats change to already spawned actors?

I tried to implement a system that allows the player to temporary slow the Ai zombies, which usually run when they see the player. But he should only be able to do so if he has acquired the according ability and hits the Zombie in the legs. My current system (Shown in the Video: Current System - YouTube) is really unreliable and doesn’t work half the time, can someone help me on how to rework it?