This is my recomendation order to solve hard puzzles, starting with the easiest one:
Super computer
Roller coaster
Surface
CGX Formatter
Genome sequencing
Bender-The money machine
TAN Network
Vendetta
skynet: the bridge
Thor vs Giants
Bender-Algorithmic complexity
skynet strikes back
clones: one step further
Indiana level 2
the labyrinth
Thanks i was about to throw myself in those but really didn’t know where to start ,since i’m not that good and i lack experience.
What would you recommend me to look into considering algorithm etc ?
I’ve started graph theory for some medium puzzles but there is certainly more than that for those
You only need to know these algorithms:
BFS, flood fill, dijkstra and A*
Good luck
Backtracking, DFS and Topological sort can come in handy as well
I solved the first three in a day , you were right @scrauler ! I just had some problems with flood fill because i used the recursive version and, well, the stack exploded ^^ but then I rewrote it to the queue version and it worked.
Until now i didn’t need the others algorithm but i’ve look into them and it looks hard for me to implement them for now ( more precisely make them fit the context ) but i guess i will learn with practice
Well done!
You must learn dijkstra algorithm before A*, because the last one is a dijkstra with heuristic.
Thanks a bunch!
The order you provided really helped me.
Before discovering your post, I started with The labyrinth and it took me about 1 week (not full-time) to solve it.
But then, I solved Super Computer, Roller Coaster and Surface in a few hours. 
I now see there are several shades of hard. 
least squares is easier for me than A*, and generalised A* is easier than its specialised variant dijkstra
Wait, what? A* is Dijkstra with heuristics, it can’t be easier and it’s also more specialised than Dijkstra (because you can’t use the same heuristics for every cases while Dijkstra never changes).
i need informed algorithm to search path from one point to another determined point, not all of them - so it is easier to understand
also, where the topic starter will put APU: Improvement Phase puzzle on this scale?
In my opinion APU:Improvement Phase should be put in Very Hard Section 
Why do you think it should be there? I’ve done 80+% of the hard section, but the very hard section is filled with really difficult puzzle, and I personally don’t think that APU improvement should be there, it’s fine in hard (at least for me).
If you disagree, feel free to explain why you think it should be in very hard section.
lol that’s fine and good but I did skynet first