The funny thing is that the first time I did this I actually finished it right away but now a day later I could not remember exactly what I did so I could not create a guide for it.
It took me quite a few times to figure out how I did it the first time around haha.

First some information about my hero and skills
This is my heroes current level at completion for this map.


These are the skillspoints I have at completion for this map.


It should be said that compared to the easy and medium difficulty you will not have the fast cooldowns for skills anymore. And you might not have all the skillpoints I have but that is less important for this map.

So I build ONLY tesla's for this. The first time this map becomes tricky is at wave 7 because there will be a few rows of worms and tesla's are not too great for that.
I focus my upgrades for tesla's in the top row and kept my hero in the bottom row to compensate.
I also kept my hero right at the front of the screen in a row where the mobs will spawn, to clear wave 7 without using your electric shock skill it is very IMPORTANT. The hero is key in this map as well as timing your skills. Use your bombs to kill things that are about to get through if your hero cannot reach them anymore.
You MUST save electric shock for wave 10 or you will be doomed.
This is what my defense looked like at wave 10.

Eventually you will have all the tesla's maxed but after wave 10 it's easy.

To recap

Skills

- Electric shock for wave 10
- Bombs to clean up stragglers
- Injections to boost the top row for example(while hero is in bottom row)

Hero
This map requires a lot of micro management with your hero.

Good luck and please rate this guide if you liked it :)