Generally first what I do is place down 2 engineer monkeys and upgrade them both to 2-0-2 or basically fast engineering and pin which will help fend off low level and basic balloon classes for a little while.
After you manage to do that try to get two 0-3-2 engineer monkeys so you can detect camo and de-armor lead balloons with the 2 upgrades on the bottom path helping to make the foam more effective.
And lastly try to place two more engineer monkeys that are 2-2-0 as the deconstruction upgrade will generally help to take down the round 40 moab.
And while your at it at the very last minute try to get all your engineer monkeys with the upgrade available sprockets to destroy groups of balloons faster.

Mostly after round 40 I usually try to go for 2 to 3, 0-2-3 engineer monkeys as the double gun upgrade will help deal with huge crowds of balloons and obviously do more dps and deconstruction helps with moabs of course.
I also place down 1 or 2 more 0-3-2 engineer monkeys to scale up with the amount of balloons every round.
At that point just upgrade all the top path and bottom path towers you can to tier 4 as sentry expert engineer monkeys can deal with more types of balloons better (Especially regrows and ceramics) and balloon traps are great for catching stranglers and halting waves.
And also you should place more 4-0-2 and 4-2-0 engineer monkeys to scale up with the balloons again and to deal with the waves of moab attacks that usually start in this round range.

For the last 20 rounds, all I really do is just make more sentry experts with deconstruction or pin depending if I need to destroy moabs better or if I need to demolish balloons faster.
And more 0-2-4 engineer monkeys for those balloon traps as halting waves is very useful for the later rounds to buy some time.

Congrats, now you know how to specifically beat hard mode with engineer monkeys