Baseball Bat Skeletons are yellow and always carry a bat. They also always drop their bat once they're defeated, so if there's a few of them on screen, knock one out and use their bat to work on the other one. These enemies frequently use charge attacks and cannot be knocked out of their wind up, but the attack is relatively slow and predictable. If you stand behind them while they attack, you can unload on them the second they stop glowing red and take them out fairly easily.
Any weapon that you can swing will count towards this, including golf clubs and guitars. You'll easily get this as these "wieldable" weapons give you a big advantage with the massive amount of damage they dole out.
Unlocking this achievement will also fulfill one of the Challenges towards completing the
Blue Belt achievement: