For hard badge on extreme mode. Select map: bag pipe. Build a rocket and max all upgrades. Then build a tight triangle of cannonx2 and taser, so that the rocket is positioned on the long side of the triangle, ie equal distance away from two of the towers. Upgrade the damage of the cannonx2 and the taser. On the other sides of the triangle, build rockets and upgrade to max. What this does is... creates three Napalm Nukes using the same cannonx2 and taser for each Napalm Nuke. It should look like this.
r /c\ r
/c t\
r
Once that is set up, start building tazers in the center bottom of the bag pipe. Max all upgrades as you go. This will slow down creeps so that the Napalm Nuke does damage to the maximum number of creeps.
 "Onslaught2")