Take the barrel out of your inventory, then use Telekinesis+2 to slam it into your enemies and instantly kill them. During combat, open your inventory menu and send the Beer Barrel to your main character. Get into a random encounter outside Cyseal. Switch back to your main character and leave the secondary character with the barrel. The barrel will overburden that character, making them unable to move. Once there, switch to your secondary character and place the barrel in their inventory. Look for a large beer barrel on stilts behind some wooden barriers and up some steps. Go on the road northwest of the market square.

After getting the Telekinesis+2 ability, go to Cyseal.