The easiest way to do this is level up the weapon stance (I think it's called 'Weapon Up'). I got it to around 35 which basically meant a bat swing was an instant kill on basic enemies.
The place to do this is on stage 2 on the rooftops. Make your way right and grab the bat. You'll get two enemies at first (variants of Williams and Linda). Kill them and you'll then get three enemies (two women, one bloke).
After that you'll get four enemies (two men, two women). The tricky thing is getting them to stand next to each other. I used LT to roll about so that I'd avoid their attacks (if they hit you, they'll usually take the bat and at this strength you'll probably kill them getting it back).
When they are grouped, swing that bat. It has a good hitting area so this is pretty doable.
I messed up a bunch of times before I nailed it. Worth playing the level through to the end as you get some upgrade mithrill blocks from Skullmageddon on this stage.