WARNING: This goes for any achievement that requires earning a medal in Single Player. If you press A to continue after completing a level too soon, you will earn the medal but you won't gain progress towards the achievement. It took me a while to figure this out, and it can be a pain not knowing which medals counted towards the achievement and which ones didn't. You don't want to have to replay every level again until you see the medal count towards the achievement. So to be safe, wait a few seconds after completing a level. I like to wait until it shows me everything I have earned for completing the mission. XP, Monkey Money, Medals and Tokens. Then I like to wait a second or two after that before pressing A to continue.
Hard difficulty is pretty difficult at first. I highly suggest waiting until you have saved up enough Monkey Money to buy Double Cash and the upgrade for your Monkey Village. I also recommend getting to Level 50 first and fully upgrading each tower by using them constantly. It's a bit of a grind, but it can be done easily by continuing on after completing a map on Beginner. Keep placing towers you need upgraded, and continue to upgrade them.
Monkey Money is earned by completing levels. Depending on your difficulty and map difficulty, the Monkey Money earned will be different. If you're in a hurry, I highly suggest searching for an online Public game and finding a map where the other person can carry you through things. This will require the other person to already have pretty much every upgrade and some knowledge of what he/she is doing.
If you should happen to find someone, completing maps on Impoppable on an Expert map, will reward you with a lot of Monkey Money. Once you can afford Double Cash, save the rest of your money to buy the Monkey Village range upgrade. NOT THE TOWER UPGRADE
Once you're set, here is my strategy that has worked for 46 maps so far on Hard and 25 maps on Impoppable.
NOTE: If you have enough money to afford Fast Tracks, do so as it will speed things up and in the long run, will save you a lot of time. Also, if a map is too difficult for you, sometimes reversing it (Pressing RT before starting), helps out a lot.
Okay, so check how many entry ways the enemies will be coming in. If it's only one, you're set to go. If it's more than one, you will have to work around certain things. This may include adding more monkeys early on, to ensure you don't lose any hearts.
If it is only one entry way, you want to begin by placing 2 Ninja monkeys near the entry way and a Apprentice Monkey in front of them. You want them to be as close to the entry way as you can get them, but leave a little room for the BloonChipper later on. Your Apprentice Monkey should be in front of the Ninja Monkeys but try to keep all 3, close together.
(If I say 2/4 or 1/3, I am talking about what your upgrade path should be. So 2/4 would be 2 upgrades on the left side and 4 upgrades on the right side. If I say 1/3, I am saying 1 upgrade on the left side and 3 upgrades on the right side.)
Begin by getting your Apprentice Monkey to (0/2). Once that's done, get both of your Ninja Monkeys also to (0/2). Once that's done, get your Apprentice Monkey to (2/2) and your Ninja Monkeys to (3/2). Once you can afford a BloonChipper, place it down in front of the 3 other Monkeys.
Now it's time to farm. Place at least 4 farms down. (If there's not enough room for 4, then place as many as you can. Make sure to keep them all close together so your Banana farmer or cursor can collect them without losing any bananas.) Upgrade each farm to (2/0).
Once each farm is done, go back and upgrade your BloonChipper to (4/2). It's important to do this before the MOAB appears, because if not, you will lose. Also get your Ninja Monkeys to (4/2) and your Apprentice Monkey to (3/2). Now it's time to place a Monkey Village. Put it somewhere near where everything is. You want it to be able to reach as much of your towers as possible as it makes upgrading them cheaper. Get your Monkey Village to (2/0).
Now you want to go back to your farms and get them all to (3/2). Once that's done, finish up your Monkey Village by doing (4/2). Now place a Super Monkey somewhere where it's not in the range of any other tower, but still covers a good amount of the map. Upgrade the Super Monkey to (1/0). Go back to your farms and finish upgrading them to (4/2).
Once your Monkey Farms are all fully upgraded, you will want yo get your Super Monkey to (4/0). It's also useful to spend a little money and place a Monkey Village next to it. You will lose the Monkey Village but save about 7,000 cash. Once the Super Monkey is (4/0), finish upgrading it to (4/2). Then make your Apprentice Monkey (4/2).
If you're following allong it should look something like this.
Apprentice Monkey - 4/2
Ninja Monkey - 4/2
Ninja Monkey - 4/2
BloonChipper - 4/2
Monkey Farms - 4/2
Monkey Village - 4/2
Super Monkey - 4/2
From here, you're set. You will still need to add more towers, but the order you do them doesn't matter. I will list off what I normally add. By the time I'm done adding and upgrading towers, I then click in the left Stick and Tap the X button while also tapping RB.
6x Sniper Monkeys - 4/2 (Place in the back somewhere)
6x Sniper Monkeys - 2/4 (Place in the back somewhere)
3x Cannon towers - 2/4 (Place in the center of the map but still close enough to the tracks)
2 GlueGunners - 4/2 (Place near entry point)
2 GlueGunners - 2/4 (Place near entry point)
3 Airplane towers - 2/4 (Place in the back somewhere)
3 Super Monkeys - 3/2 (Place near the track, close to entry point)
3 Super Monkeys - 2/4 (Place near the track, close to entry point)
2 Mortar towers - 4/2 (Place target at entry point)
2 Mortar towers - 2/4 (Place target at entry point)
5 BloonChippers - 4/2 (Place together near entry point)
1 Spiketrap - 2/4 (Place near entry point)
2 Spiketraps - 4/2 (Place where you think would be best)
If you still have room, keep adding whatever else you may find useful. Good luck. Seriously, with Double Cash this is possible. I'm not sure if this method works without it.