I've never been good with slider puzzles ....
Any strategy suggestions to finish the "bigger slider puzzle" in the hidden cave?
Got the 2 top rows done, but the 2 lower ones are giving me a headache.
Can anyone help?

I confess, I do that daily, just for fun, and I ran into a problem one time. If you have the top two rows done and can't get the last two, go back and check that you haven't got two of the pieces, especially 2 and 3, in the wrong order. If you do, the last rows won't fall into place. If you don't, remember, the pieces slide up and down as well as back and forth so take the tine to see which pieces need to be put where and they will eventually fall into place.

does anyone know what the counter is counting down too after you complete the large slider puzzle?

Thank you Raymond, that was exactly what I needed!
And thanks to everyone else, too.

I just got my puzzle done in under 100 moves... Some was luck because of how the tiles were arranged in the beginning... time will tell how much I learned by working the slider puzzle daily.

I have a little square of numbers next to my computer and I try to get the squares to head in the right direction and trial in a good order....

1 2 3 4
5 6 7 8
9 10 11 12
13 14 15

and the snake like orders are
5 1 2 3 4 8
1or9 5 6 7 8 4or12
9 13 14 15 12 11

not sure if that will describe what I am doing or not but there it is

People who play it, and not use the helper apps, learn the 'standard' moves.
Basically little 'snakes' of 3 or 4 numbers that will fill a row. And clockwise or counterclockwise snakes depending on if it's moves to fill from the left or the right. That last sentence is critical for the last 2 rows.
The only problem with this is that sometimes it's not possible to do it in under 100 moves. There are 'other' semi-standard moves that working 2 rows in conjunction can make it work. But in actuality it all depends on the randomness of the starting position, sometimes it's really randomized and sometimes it's not so random.
p.s. Nobody can get it done in exactly 100 moves, it's always an odd number of moves.

also helps to look for tiles that are in (or nearly in) the correct order. Take care because some will be in reverse order once they are moved into place.