You need a least one scientist in your team (which you have if you didnt change the default team member setting). Start up the level "Meat in the Middle". You can kill all the enemy worms, but don't finish the level by collecting the last crate. Collect the health pack with your fourth team member and always use the "Skip Go" rope to skip your turns. Each time its a scientists turn your team members will get +5 health. Rince and repeat until your 4th worm has reached 300 hitpoints.