I have now looked at the first 14 levels of Vicious, all done apart from Changing of the Guard.

Vicious  levels 1-6 - First 5 all felt like a jump down in difficulty from most of Daunting. , but that said the first few of each difficulty so far have felt easier than the end of the previous one.

Dividing Three By Two - I like the puzzle to this, the part of the solution I missed for a while was
Conundrum - This one didn't take long. I feel the main element to this could have been used as part of a bigger level. 
Endeavor  - Aside from the US English spelling this is up there with my favourites so far. It had 2 elements to the solution that I have not used before
Low Profile - Completed first attempt, feels more like late Quirky/early Cunning in difficulty. I don't mind having a breather every so often mind

Betcha Can't Save Just One - A few things about this level have me curious. Firstly, is it possible to complete this in any of the 4 sanctuaries? Secondly, the title. It implies to me that the setter is saying saving just 1 is not possible, you have to save 2 or more? Betcha Can't Even Save One as a title would give the impression that you should target 1 only. Finally

Changing of the Guards - Errmmmm.... I don't even know where to start with this one. When I see the solution I'm either going to think I missed something really easy or that its an ingenious level. Maybe both. Not asking for hints yet, I don't even think I've scratched the surface with this!

 

Those are all good hints for general problem solving - many are ideas I try to consider when stuck. Linked with skill management, I'd also add that working backwards can help - if you know you can solve a level by getting all lix to a particular place then the problem turns into "how do I get there" and is simplified.

The one I am worst at dealing with is spending ages pursuing a red herring path that almost but not quite works.

To Destroy is to Construct - I liked this level, took a while to figure out
Two For His Heels - My best attempt is attached. I can see how to get an extra turner if that helps, but I am either a digger or miner short and don't see how to get that.
The Lix Who Japed - Took longer than it should have
Just Stop the Bleeding - Took a few attempts but mainly a case of working out how few skills you can get away with using on each side.

Cornerstone - Really liked the design of this, made me feel dumb for not getting thow to do it at first then smart when I worked it out! I think the hallmark of a good level in general is that it should either look really simple and actually be tough, or look horrible until you see something to simplify it. This fulfills the first of the criteria very nicely.

Repeating Patterns - As a maths teacher I approve of the fractal. Nice to see a level where you need to carefully think about where you use your turners.

The Crimson Room - Looks easier than it is I think - seeing where to save on skills to have enough I found quite tough. This was one of the later levels I completed. 

Narbacular Drop -
Duality - I don't know if my solution is intended but can't see another
BOGOF part 2 - Nice puzzle, I like that the differences to BOGOF 1 in skills are subtle yet make for such a different solution.

Alternative Route Required - Trailblazer fare with a limited skillset to choose from.
This Lix is Your Lix -
Tribute to Benny Hill - My attempt is below, it saves only 4 lix though is a couple of skill short of saving all of them. I think I have some elements of the solution correct but am missing something. Feels significantly tougher than all the levels around it, though it could be a very easy one where I am missing something simple. Hints would be appreciated at this point  
Railroad Plot - Seemed relatively easy for the difficulty, this only took a couple of goes.

Derailed Level - On the other hand this one took a lot of attempts.
Lixes in Motion - Nice timing puzzle.

On the Daunting set:

I haven't completed them all yet, there are 7 left to do, all of which I have looked at but can't do yet - namely Tribute to Benny Hill, Derailed Level, I'm Gonna Make you Mine, Passing Engagement, To Destroy is to Construct, Two For His Heels, and The Lix Who Japed. So I suppose they are the ones I think are the hardest!

Will post 6 levels at a time with comments and my solutions.

Rhapsody - Nice and relatively easy opener, just needed a few tries to see which skills were employed in which places.

Round Trip - Completed 1st attempt, the easiest level in this set?

Death or Glory - Took a few goes to see what was going on here, I like the design on this one.

Over My Head - For me, it was for a while! This one is up there with my favourites for daunting. 
Backslash - Minimalist level showing a useful trick, I like it.
Now This is Fun - Bit of a slog this one, I feel it could have been compacted a bit whilst maintaining the same puzzles? My replay looks very un-optimised, I'm sure I could have used fewer skills to solve it in a similar way.

A few other level attachments - Lix Cannon and LMP3 both felt a bit fiddly with how I did them, and I am curious as to whether my solution for Under the Rainbow is the only one