How do you i beat level 6 in bloons tower defense 4 on hard for the iphone?

To beat level 6 in Bloons Tower Defense 4 on hard mode, focus on placing strategic towers like the Dart Monkey and the Tack Shooter near the start of the track. Upgrade your towers as soon as you can, prioritizing faster shooting and increased range. Additionally, consider placing a Spike Factory at the end of the track to catch any bloons that get past your defenses. Manage your resources wisely to ensure you can handle the increasing wave difficulty.