On each level, you get a gold medal if you finish first time, or on your first attempt since you last tried for a medal. The next attempt or two (depending on the level) will get you a silver medal, and thereafter it's worth bronze. There is a reason for this: on attempts worth gold medals, you'll get hardly any help in terms of crates. Silver will give you a bit more help, and bronze a bit more still. Thus getting a gold medal really is harder than getting a silver or bronze, in many cases. When I say below that you "get" something on a silver/bronze attempt, it means that there'll be a crate containing it.
11: Chemical Warfare
Difficulty: 4
This level is fairly straightforward when you get the hang of it. You should poison all the worms you can't reach (as the landscape's indestructible) as soon as you have the skunk. (Until then, use any infinite weapons you have such as the firepunch and uzi to do damage. The worm in the bottom right corner is ripe for a kicking as soon as possible.)
When the poisoned worms have lost all but one point of energy each, start using the shotgun on them carefully. Following these rules, you should have no problems, although it's worth getting under cover, as the enemy will be using plenty of airstrikes.
12: No Substitute
Difficulty: 5
Another general hints level, this one:
The key to this level is getting the high ground - i.e. somewhere on the tree - and keeping it until everyone else has drowned. The actual top of the tree is rather dangerous as it's easily to get chucked off it. Having said that, it's good to use a girder so that you can get on top when (say) all the enemies are under the tree. I've found that in practice it's the worm that starts under the tree that is most likely to survive. The homing missile can be really handy in this mission in various places.
13: Who left the flood-gates open?
Difficulty: 7
In this level, I always pretty much forget about the left hand worm: it's very hard to save both worms, and the one on the right has less competition. Both goes below refer to the right hand worm.
Go 1: Go onto the bit of land on your right, vaguely close to the pipe on the right. You can skip the rest of the go. With any luck, the worm on your right will now fire a bazooka which will nearly open up the way.
Go 2: Use the shotgun to get out, using the spare shot on the enemy.
Go 3: Get as high as you can, and place a girder to stop yourself from being knocked into the water.
Go 4: Ninja up to the far right hand corner.
From here, you should play it by ear to a certain extent. However, you can assure victory by building girders up to the right hand edge of the yellow thing in the middle at the top. You can make a nice little hidey-hole, and you'll be higher than anyone else on the level. Then you just need to wait for the water to rise.
14: Super Sheep to the rescue!
Difficulty: 6 or 1, depending on how lazy you are and the version
you've got.
If you're really good with the Super Sheep, you can fly it all round the level and get the crate. Alternatively, with the non-US version, you can win the level instantly by killing yourself with the sheep. You will blow up the enemy (if he isn't already dead), and draw the round - which completes the mission. Simple, eh? I'm afraid that if you've got the US version you'll have to do it properly... but it's not actually that hard.
Difficulty: 6
There are three enemies on this level. The top one can by blown into the water using dynamite. The remaining two are the problem... however, you should be able to get them together using the shotgun. If you possibly can, you should then collect the utility, and drop the Holy Hand Grenade on them in the same turn. Don't sorry about pinpoint accuracy or your own safety - you'd be hard pressed to get clear of an explosion that large.
If you can't do it quite right (due to the mine, or whatever), don't worry... the HHG makes a large hole in the landscape anyway, after which the sheep and/or firepunches can finish the job off.