Give your monster_gman a name first. Then, draw an invisible, null-textured brush around him and tie it to func_breakable. Set whatever properties you want, but set the "Material type" field to "Flesh". After that, also set the "Target on break" field to "kill_relay_1", this is going to be the name of our trigger_relay. Create it first, then set the "Name" field to "kill_relay_1". Then type your monster_gman's name into the "KillTarget" field. You may also turn the trigger_relay's "Remove on fire" flag on so it gets removed once it has been activated.