This is done in the first level "The Village"
You will need to take out 10 enemies and not get hit the entire time.
The easiest way I found to do this was to run to the bottom left corner as quick as I could as Bud. The enemies can't hit you down here. Let AI Terence take out the enemies which could be a bit.
After 5 enemies are defeated you will switch to Terence. Try your best to immediately run into the same corner. I found that occasionally on the way over an enemy in the top window would hit me, if so just restart. Once you get in the corner try to pick up some jars between the enemies throws. The enemy in the top window can occasionally hit you even when you're in the corner so prioritize taking them out. Hold throw and the aim reticle will lock over an enemy so you can take out this second group rather quickly.
Keep an eye on your health and it will pop up if nobody hit you the entire time.