You're not alone - those are the ones I've found hardest, and I'm #1 in the game centre rankings today (Unless Ken's topped me again)
A few level tips which I hope might help (but please note that I'm on 4th difficulty - ie. highest, and it may be its a little different due to extra coins being dropped allowing extra funding).
1) Stream of happiness - It took me several goes to find a system that worked - I've not found a layout that worked without the first tower being a red aoe tower on the top left loop. Also, as required I put red aoe towers on the two other "thinnest slots", such that they can attack both sides. Blue tower, early I put it on one of the middle slots so it can attack the blues lots of times on their way through. Note this level I often had to use power-ups to get the last beasties.
2) Blueberry pond - a little bit of "memory" of what is coming is important here, mainly to remember when the speedy blue jellies are about to come. I also found that a blue zap tower (the most expensive one) is good at the top right (lower slot) to zap early on. Harder difficulties also require a second blue zapper down bottom middle (where left route and middle route meet). The last 4 slots before "its too late" I generally have the 100 cost blue/red tower (the very last slot needs upgrading once to get jellies coming both routes)
3) Frosting Immensity - This one I always hated as its hard to work out what's coming from where. Trial and error fixed this one for me, by adapting the youtube run through a little bit (an extra tower here and there to deal with the extra jellies). Note you mustn't use the tornado for anything except the jelliphants, unless you are happy building lots of blue zap towers just for one group.
4) Whipped cream soft core - The reason I'm at the top slot currently. The lt.general can boost the bank, which is why I can get silly scores (in fact, I could get a lot better score if only the iphone didn't crash at just over 30 million coins!). BUT - how did I complete the level? Very painfully, starting by writing down what comes each round, and then plotting a strategy accordingly. A couple of points though - i start with a red tower at the top, and two blue towers (one just below the red tower, and one right at the end in the middle), and researching a bank. I place bank asap. When the second round of red blob jellies comes round (As I recall, it goes red blob jellies, red speedy jellies, red blob jellies) I have to time it just right, upgrading the tower to level 2 (ie. the upgrade costing 100). If you time it right (such that the upgrade finishes just after the jellies get into range), the tower lets off two bombs quickly. If you don't do this, the tower doesn't kill all of them itself, and the red jellies left walk for miles and miles (mean time others come, you have to place more towers, and you get less gold from bank).
Later on, when blues come, you have to get them in the right places to get everything correctly. I'm normally upgrading two red towers (at one below maximum) plus 2 blue towers (same state), plus bank. There's some blue/reds, less important. Once you use the "power upgrade", you'll be selling the blues soon afterwards, and the money starts rolling in. Note also - For the first half of the game, I have to use every power-up at just the right time. You also need to be researching quite heavily as you need the electric tower in time to get some speedy blues, and the blue zap tower before the second round of jellyphants. An evil level - and absolutely great feeling when you work out a strategy
. I'll see if I can do a level four difficulty video at some point, I just don't want to give away all my secrets to loose my top slot
5) Fruit sherbet bridge - On level 4 difficulty, I can't do this level without waiting on the bank. What I mean is, every other level with bank being available, I can place it asap. This one I can't, it's much later before it goes down. I have to use every penny getting going. That tip aside, I basically adapted the runthrough already available on youtube.
Some general comments as well:
1) Towers upgraded will fire twice if timed right. Towers not upgraded will not. This doesn't apply to the bank (I don't believe?), but instead note if you get it wrong timing, the bank gives no coins at all for a round! Best with the bank to not upgrade when a group is about to die.
2) Blue machine gun tower fires a little bit quicker when the game is put on fast. Enough to beat a round of blue jellies when it wouldn't on normal. This is critical to beat some for me. Not sure if this is deliberate or accident (please don't change it - it's the only way I can beat some levels!)
3) While I'm at number 1 because of one level (Whipped cream, I have 150 million +), I only average about 50th on every other level. J just a comment that I've beaten them all but others obviously have better ways of doing most levels.
Mods - I assume its okay to give tips in this way? It's not against rules?
All the best,