Step 1- Get Sickle
Step 2- Stand next to Pack-a-Punch
Step 3- Have 2 or more people knife you or you knife someone else ( people must have sickles and 1 person with upgraded sickle and ballistic)
What's wrong with Upgrading a Thunder Gun on round 14?
Its not bad but some people don't upgrade thunder gun / ray gun until they are completely out of ammo and don't think a max ammo is coming anytime soon. I upgrade thundergun at like round 27 and ray gun at like round 33, but thats just me.