LVL 15 advice. I found the final boss pretty challenging. I would highly recommend fully upgrading your hero and allies, and purchasing all runes. I probably wasted $100K before I truly appreciated that if you continue hitting him. He's isn't able to send a wave of enemies. Hypothetically, you can beat the boss without having to kill any other enemies. YOU MUST STAY ON HIM. When he is on his knee bash him as much as you can. As soon as he teleports break away from your allies to be able to cover as much ground as possible. If he warps to the other side don't be afraid to blindly shoot X. Anything to keep a wave of enemies coming. I hope this helps. Please rate up to share.
to quote monsterenergy "Hard Badge: Day 1: Upgrade the engine, transmission, and tires to level 3, rest goes to gas. Day 2: Max out the gas, rest goes to the gun. Day 3: Max out the gun, rest goes to boost. Should be pretty easy with a little bit of skill." As far as the skill part. Its all wheel drive. Keep the wheels on the ground. Lean back when you approach zombies. So that you pass through their heads, and keep momentum. I did this ten times, until I ran out of gas at the boxes and boosted through them(momentum!), but you have to have good enough runs on day 1 & 2 to afford enough boost to do it.
 "Mighty Knight"){kind=icon}
 "Nightmare Runner"){kind=icon}
 "Earn to Die"){kind=icon}