Here is a fail-safe walkthrough that will help you beat all the beginner levels on Bloons Tower Defense 5 on the HARD setting.  The strategy will need some tweaking to work for the intermediate, advanced, and expert levels.  This is but one possible strategy to win Bloons Tower Defense on Hard.  I do not use any abilities, farms, or super monkeys.   My solution here is based on the Android version.  For Windows and other platforms, my Bloons TD5 strategy will work very much the same, but there could be some slight differences as I have not tested outside of Android.  Get the Android Bloons TD5 from the Play Store here.

I have played a couple of co-op online games and then realized that a lot of players play this tower defense game without any strategy at all.  They just build and break at random.  Not knowing what works and how to defend to be ready for the next level.

I have tested this strategy successfully on all the beginner levels that were available at the time of posting (monkey lane, 3 times around, north pole, patch, maze, fireworks, brick wall, z factor, park path, the rink, roswell, and space truckin.) 

Use the below build order and guide.  I will use the screen-prints on the monkey lane board.  To simplify this walkthrough, I have only built towers in between rounds. 

Round 1:  Build a Dart Monkey.    Place the dart monkey close to the start.  The optimal location will be where this same dart monkey can target the track much later on again.   This is a good location for monkey lane

Upgrade the dart monkey to Long Range Darts, Enhanced Eyesight, and Sharp Shots.

Go!

Round 2: Upgrade your dart monkey to Razor Sharp Shots.

Round 4 end: Upgrade your dart monkey to Triple Darts.

Round 12 end: Place a Sniper Monkey. For now, this monkey will take car of anything that tried to escape.    I placed it here, but you can place it anywhere as its range will be the full screen.

Upgrade the Sniper to Full Metal Jacket and Faster Firing.

Round 24 end: Build a Monkey Village.  You need to place it so that it covers your glue gunner, and there must be space around it to build cover a couple of Bomb Towers.

I placed it here:

Build a Glue Gunner.   The placing is very important.  You need to place it close to the start, have a big open area - we need space to place a monkey village with lots of towers next to it - and it must cover some track further down.  This is where I place it in Monkey Lane:

Upgrade the glue gunner:  Glue Soak, Corrosive Glue, Stickier Glue.   Change the glue gunner so that it target strong.

Round 25 end: Upgrade the Monkey Village to Monkey Beacon.  This will increase the attach range.

Round 27 end: Upgrade the Monkey Village to Monkey Fort.

Round 32 end: Upgrade the sniper to Night Vision Goggles.  He can now detect and shoot camo balloons. (Hint: This is needed for round 33 with a group of Camo bloons.  The single dart monkey sometimes are not strong enough to take them all out).

Round 33 end: Upgrade the Glue Gunner to Glue Splatter.

Round 36: Upgrade the Monkey Village to Radar Scanner.  This is important so that the towers under the village can take out camo balloons.

Round 38: Build a Bomb Tower under the cover of the Monkey Village.

Upgrade the Bomb Tower: Extra Range, Frag Bombs, Cluster Bombs, Bigger Bombs.  Target First.  In Round 39, upgrade the tower to Missile Launcher.

Round 41 end: Upgrade the Bomb Tower to Bloon Impact.

Round 42 end: Build a Bomb Tower, also under the shade of the Monkey Village.  Hint: If you are not sure if it is under cover, have a look at the range of the tower.  The range will drop when you move too far away from the village.  Upgrade this tower to Extra Range, Frag Bombs, Bigger Bombs, Missile Launcher.  Set to attack Strong.

Round 43 end: Upgrade this new Bomb Tower to MOAB Mauler.

Round 45 end: Build another Bomb Tower, same as the previous one.  Remember to set to attack Strong.

This configuration will be strong enough to take out the blue MOAB balloon that will attach you in round 46.

Round 46 end: Upgrade the Monkey Village to Jungle Drums.  This will increase the attack speed.

Round 48 end: Upgrade the Glue Gunner to Bloon Dissolver.

Round 49 end: Build a Bomb Tower at the Village.  This one I place so that it covers the furthest away track - but still under cover from the Village.  Upgrade Extra Range, Frag Bombs, Cluster Bombs, Bloon Impact, Bigger Bombs, Missile Launcher.  Target first.

Round 50 end: Build a Bomb Tower and upgrade to MOAB Mauler with Target Strong.    (Upgrade Extra Range, Frag Bombs, Bigger Bombs, Missile Launcher and MOAB Mauler).  Build at the Village.

Round 52 end:  Build another MOAB Mauler tower as the previous.

Round 54 end: Build another MOAB Mauler tower as the previous.

Round 55 end: Build another MOAB Mauler tower as the previous.

This is what we have now:

Now, keep on building MOAB Maulers as above as soon as you have enough money.

When you have 8 MOAB Maulers, then build another Bloon Impact with target first.  This tower will really help when the big field of ceramics and camo's attack.

At the end of Round 70, I had 13 MOAB Mauler- and 3 Bloon Impact Bomb Towers, all under the Monkey Village. 

No more space for towers under the Village.   So, now you can start another Monkey Village and only need to build MOAB Maulers.  You need a couple more to solve the last Hard Round (Round 85).  Upgrade the Monkey Village same as the other one - you will appreciate the Radar Scanner in Round 82!  So, upgrade Monkey Beacon, Monkey Fort, Radar Scanner, and Jungle Drums.

At the end of Round 80:  I am almost ready for the final level!

Round 84 is a tough one.  Some balloons almost made it!  The far-end Bloon Impact Bomb Tower did it for me.

Round 85... a bit easier than 84!  No match for my towers.  I won before this big green machine go half-way

Hope you enjoyed this walkthrough for Bloons Tower Defense 5.

Please comment bellow and share your experience, tips, tricks, and help.