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.
21: Wooden ambush
Difficulty: 5
This level can be easy or a pain, depending on a bit of luck near the start. The enemies on the left will blast through to one of your worms, and if you can drowned them without too much of a problem, you should be able to do the rest of the level easily. If they're persistent, you may have more of a problem. Don't forget to fire mortars behind their heads for maximum effect, or at their heads from above. You may wish to use the ninja rope to knock them down the slope a little before firing at them from the top of the enclosure. Even if you don't kill them in the first shot, you should then be able to survive and kill them in the second. (Note: according to the patch notes, you can't knock worms around with the ninja rope when the patch has been installed. It may still be best to go up the slope and fire down at them, taking the consequences.)
Although you can take the level fairly slowly, the enemy gets airstrikes after a while, so be careful.
A crate containing two old women near the middle (just above the pumpkin) - this is really useful against the worms on the right.
The jetpacks are really handy, but don't be too ambitious - they don't have much fuel.
You must finish this level before the earthquake chucks the crate into the water. Here's how:
Go 1: Move your worm onto the top of the grape stalk, and fire a bazooka just under Sentry. This will hopefully kill him. As soon as you've fired, jump onto the apple. You'll take minor damage, but don't worry.
Go 2: Leave a grenade at the point where the stalk of the big apple meets the leaf (i.e. the really thin bit) - then get out of the way! This should provide you with a route to the bananas.
Go 3: Blowtorch into the banana.
The enemy will hopefully now fire a bazooka into the banana, from the other side.
Go 4: Retreat a bit, then fire a bazooka down your blowtorch tunnel to make a path through the banana.
Go 5: Parachutes should now be enabled. Walk up to the top of the second banana, activate parachutes, and jump to avoid the mine. When you're past the mine, hold down left to pick up the crate, and the mission is done.
23: The Drop Zone
Difficulty: 5
I used to think this level was a complete pain - but that was before I learnt to use the parachute properly. As well as left and right, you can use up+left to really slow your descent. If you want a real challenge, try the level without using the up key during parachuting...
Okay, so you need to parachute down onto each worm in any order, and blow them up. Firepunching the rightmost worm left kills the one next to it - otherwise you wouldn't have enough suicide bombers (although the mine by the third one can kill it). If you fail a few times, you can use a shotgun for two turns, which makes it much easier, but you'll only get a bronze medal. Use girders to extend the cliff if the enemy blows up the end of it. Use the furthest worms for the rightmost enemies, as you won't have enough time to get them all the way over to the left... also, they're the most likely to die, so you should use them early.
Thanks to Koyser [dlb] for explaining the parachutes to me - shame I'd already got a gold on it after hours of trying!
24: Countdown to Armageddon!
Difficulty: 5 - 2 with the guide
Many thanks to Smoke for this suggestion - it works a treat!
First, ninja over to the mayor, and baseball bat him, having aimed as high as possible and to the left. This will take away most of his energy, but leave him in a nice deep hole.
In your next go, you should get over to the mayor on a ninja rope and girder both of you in.
After this, you have plenty of time. A couple of girder starter kits appear after a while. Pick them up with your last ninja rope and place girders all around the hole - especially over the top, obviously! You will almost certainly survive the attack, and hopefully the enemy will die at the same time. If not, just pick them off with whatever worms you've got left.
This one's dead easy. You may be used to the landscape from the Super Sheep Racing training, but the sheep you have in this mission doesn't fly. It does, however, jump - and it's got to jump lots.
Use your girder starter pack, laying two girders between your worm and the second block, one girder (slightly lower than the top of the landscape) in the next gap, then two girders for the last gap (just to be on the safe side). The first girder should be placed right next to the green spaceship, so that if the sheep falls back it will keep going.
Now launch your sheep. If you've placed the girders correctly (and you're lucky - I've had some really irritating goes when writing this guide), the sheep will run all the way to the right of the terrain. Detonate it at the bottom of the rightmost strut (when it lands after its last jump), and it will blow up the oil drum, which will in turn blow up the crate.
If you miss with the sheep, you can try using the grenades, but I've never managed to blow it up, even when the grenades come pretty close.