BTD6 How to Black Boarder ANY Beginner Map
This is a guide on how to black boarder any of the beginner maps with little to no trouble. (maybe even some intermediate maps) Anyways I hope this was kinda helpful.
TLDR: first use a cheap monkey (my recommendation would be druid and get one upgrade on the 2nd path) or sauda (might need to get an extra monkey if the map isn't optimal) as your first tower. Then work towards getting a Helipad and as long as you progress on the first path, it should be done and dusted.