If you really want to make minions out of creatures, it’s easy enough to give creatures 1 hp and say only direct hits deal damage.
If 1-hp creatures aren’t your thing, you could still do a version that skips keeping track of hp: Any direct hit that deals more damage than creature has Hit Dice automatically kills. Any hit that does less than that threshold adds dealt damage to 10 to set a Con save DC for target to make. Success = survival.
Example: Beck d’Corbin attacks a minionized ogre with two polearm attacks and a bonus attack with the other end. The ogre has 7 HD (and an AC 11, Con save +3), so a hit with Great Weapon Master’s +10 damage would be an automatic kill, but Beck is still doing fine without.
Attack 1 misses and attack 2 rolls minimum damage on a d10 + STR (let’s say, +4). Since 5 is less than 7, the Ogre must make a DC 15 (10 + damage dealt) Con save to shrug off the lethal blow. Which it does. (Note that the highest that DC could get would be 17, if the damage had been 7.)
Fortunately for Beck, the bonus attack lands max damage with the 1d4 +4 and that means 8 vs. 7. Automatic death. On to the next monster.