I have tested this strategy on beginner and a few intermediate tracks and it worked every time. I hope it works for you too!

Requirements:

• All the towers & upgrades I'll mention (you should have them if trying to beat impoppable anyway)
  
• A fully upgraded Engineer Workshop

Buy an engineer to begin with. Try to place him somewhere near the mid/start of the map. Take advantage of his range to hit as many bloons as possible.
After that, upgrade him to 2-3.
When you are done upgrading him, buy a Buccaneer. As always, try to take advantage of his long range. If the track has no water, try using a portable lake.
Upgrade him to 0-2 because the camo bloons will come soon! (Only one will come, but it will be enough to make you lose in impoppable mode)
After that, buy 3 upgrades on path one, so it'll be 3-2.
Buy a Banana Farm and make sure to place it somewhere where you can add many more nearby. Upgrade it to 2-1.
After that, buy a monkey village and place it near the engineer. Upgrade it to 0-2.
Gather money and buy a super monkey near the village. Upgrade it to 2-3.
After you do that, upgrade the banana farm to reach 4-2 (the order in which you buy the upgrades doesn't matter)
At this point, you should have a 4-2 banana farm, a 0-2 monkey village, a 3-2 buccaneer, and a 2-3 engineer.
Then, buy a monkey ace and place it in the radius of the monkey village. keep upgrading it till you reach 4-2.
It should then be easy to hold off the bloons. You can buy everything you want! A few suggestions:

• 2 more banana farms (for a total of 3)
  
• A heli pilot
  
• Another engineer
  
• A sub
  
• A mortar
  
• Pretty much every tower in the game

Round 85 should be easy, even in impoppable.

I hope I helped you with my strategy. If you have any questions or suggestions, tell me in the comments! Good luck with your bloon hunting journey!